Cardiovascular disease
Higher riskstrong evidenceWhy heat is harder
- Heat dilates skin blood vessels, so the heart must work harder to hold blood pressure up.
- Sweating reduces blood volume, adding strain on a heart with limited reserve.
- Heat thickens the blood and raises clotting tendency, both linked to acute coronary events.
What helps
- Avoid exertion between 11am and 3pm, and rest in the shade instead.
- Cool down with cool showers and damp cloths rather than relying on a fan alone.
- Go easy on alcohol on hot days. Tea and coffee still count towards your fluids.
Sources
Heart failure
Higher riskmoderate evidenceWhy heat is harder
- A failing heart cannot raise its output enough to supply the extra skin blood flow that cooling needs.
- Standard treatment restricts fluid, which pulls against the extra fluid loss caused by heat.
- Diuretics add to fluid loss on top of sweating, and dehydration itself worsens symptoms.
What helps
- Ask your GP or heart failure nurse whether your fluid limit should change in hot weather. Do not change it yourself.
- Handle thirst within your limit using small sips and ice chips.
- Learn the difference between dehydration signs such as dizziness and dark urine, and fluid overload signs such as swelling and breathlessness. Report either.
- Keep weighing yourself as usual so changes get picked up early.
Sources
Atrial fibrillation
Moderate riskmoderate evidenceWhy heat is harder
- Sweating depletes potassium and magnesium, which the heart needs for normal rhythm.
- Dehydration thickens the blood and raises autonomic stress, both known rhythm triggers.
- Hospital admissions rise at temperature extremes, though the cold effect is bigger than the heat effect.
What helps
- Keep fluid intake steady across the day rather than in large bursts.
- Remember diuretics add to fluid and salt loss in heat.
- Go easy on alcohol, which dehydrates you and can trigger episodes.
- Get new or worsening palpitations checked.
Sources
High blood pressure
Higher riskstrong evidenceWhy heat is harder
- Heat lowers blood pressure by itself, so with blood pressure tablets the drop can become too large.
- Diuretics increase fluid loss through urine on top of losses from sweating.
- ACE inhibitors and ARBs reduce the kidney's ability to protect its own blood flow, so dehydration raises the risk of kidney injury.
What helps
- Keep fluids up, and do not stop any medicine without medical advice.
- Ask your GP whether your diuretic dose needs a review during a heatwave.
- Watch for dizziness, confusion, cramps, passing less urine or a racing heart.
- Store your medicines below 25C, away from sunlit windows and the car.
Sources
Previous stroke
Higher riskmoderate evidenceWhy heat is harder
- Damage to the hypothalamus or brainstem can directly impair control of body temperature.
- Reduced mobility makes it hard to move somewhere cooler or change clothing.
- Aphasia and other communication problems can stop someone reporting that they feel unwell or too hot.
What helps
- Family and carers should check in rather than waiting to be told there is a problem.
- Agree a simple way to signal discomfort, such as cards or an alert device, if speech is affected.
- Offer drinks regularly instead of leaving someone to ask.
- Avoid unsupervised trips out in peak heat if mobility is limited.
Sources
Chronic kidney disease
Higher riskstrong evidenceWhy heat is harder
- Reduced ability to concentrate urine leaves less margin against fluid loss from sweating.
- Dehydration cuts blood flow to the kidneys, raising the risk of acute kidney injury on top of existing disease.
- ACE inhibitors, ARBs and anti-inflammatory painkillers all interfere with the kidney's blood flow control.
What helps
- Know your sick day rules: some medicines are paused temporarily if you are vomiting, have diarrhoea or cannot keep fluids down.
- Avoid ibuprofen and similar anti-inflammatory painkillers in hot weather.
- Contact your kidney team early if you notice dark urine, passing less urine, dizziness or unusual tiredness.
- Skip sugary drinks, alcohol and heavy caffeine, and stick to plain water.
Sources
Kidney stones
Moderate riskstrong evidenceWhy heat is harder
- Fluid lost through sweat makes the body concentrate urine.
- Concentrated urine lets calcium and uric acid salts crystallise more easily.
- Stone and renal colic rates rise in warm seasons, and admissions climb with monthly average temperature.
What helps
- Drink enough to keep urine pale rather than waiting until you feel thirsty.
- Replace sweat losses with water rather than sugary or caffeinated drinks alone.
- If you have had a stone before, be extra deliberate about fluids during a heatwave.
- Use urine colour as a quick daily hydration check.
Sources
Dialysis
Higher riskmoderate evidenceWhy heat is harder
- With little or no kidney function, extra fluid cannot be passed out and builds up until the next session.
- Heat increases thirst and sweating, but drinking still counts against a fixed fluid allowance.
- Heavy sweating without salt replacement can cause low blood pressure and cramping during dialysis.
What helps
- Stick to the fluid allowance your renal team set, and tell them when a heatwave is forecast so they can review it.
- Manage thirst within your allowance using ice chips, chewing gum or cold water mouth rinses.
- Track weight and blood pressure more closely in hot spells and report unexpected changes.
- Report dizziness and cramping, and also breathlessness or swelling, as these point in opposite directions.
Sources
Type 1 diabetes
Higher riskstrong evidenceWhy heat is harder
- Heat increases skin blood flow, which can speed up insulin absorption and bring on low blood sugar.
- Insulin degrades faster when stored above its recommended temperature, and meters and sensors can read inaccurately in heat.
- Dehydration raises the risk of diabetic ketoacidosis, especially if you are also unwell.
What helps
- Check blood glucose more often, as lows can come on faster than usual.
- Keep insulin and kit out of direct sun in a cool bag, never in a car or against a freezer compartment.
- Carry extra hypo treatment.
- Follow sick day rules if unwell: keep taking insulin, test for ketones, and get help if ketones rise or you cannot keep fluids down.
Sources
Type 2 diabetes
Higher riskstrong evidenceWhy heat is harder
- Nerve damage from long-standing diabetes can reduce sweating, so the body cools itself less well.
- Blood vessel disease reduces skin blood flow, limiting heat loss further.
- SGLT2 inhibitors add to dehydration risk, and metformin carries a small risk of lactic acidosis if dehydration or kidney injury develops.
What helps
- Know your sick day rules for SGLT2 inhibitors and metformin if you become unwell or cannot keep fluids down.
- Drink more water proactively, since thirst can be a poor guide.
- Check blood glucose more often during hot spells.
- Protect your feet: reduced sensation plus hot pavements, sand and car parks can cause burns without you feeling it.
Sources
Thyroid disorders
Moderate riskmoderate evidenceWhy heat is harder
- An overactive thyroid raises metabolic rate, generating more internal heat and causing heat intolerance.
- Levothyroxine replacement raises metabolic demand, and UKHSA lists thyroxine among medicines needing extra thought in heat.
- Poorly controlled overactive thyroid carries a risk of thyroid storm, which heat stress can contribute to.
What helps
- Do not change your thyroxine dose because of the weather without medical advice.
- Report new heat intolerance, tremor or palpitations, as these can signal a dose that needs reviewing.
- If your thyroid is overactive, be especially careful about hydration and cooling.
- Store medicines below 25C and out of direct sun.
Sources
Adrenal insufficiency and Addison's disease
Higher riskmoderate evidenceWhy heat is harder
- Sweating loses sodium and fluid that the body cannot compensate for, because aldosterone control is impaired or absent.
- Usual steroid doses are set for typical conditions, and heat raises the body's demand for cortisol.
- Vomiting or diarrhoea from heat illness stops oral steroids being absorbed, which can tip you into adrenal crisis.
What helps
- Follow your sick day rules if you feel unwell or feverish, even without an infection.
- Carry your emergency hydrocortisone injection and steroid card at all times.
- Speak to your endocrine team about whether fludrocortisone needs adjusting if you feel light headed in heat.
- Include salty snacks or rehydration salts, since plain water does not replace sodium.
Sources
Obesity
Higher riskstrong evidenceWhy heat is harder
- Body fat insulates, slowing heat loss from the core out to the skin.
- Higher resting heat production, and less surface area for each kilogram of body mass, both reduce cooling.
- Common related conditions and their medicines further impair temperature control.
What helps
- Prioritise cool, shaded or air conditioned spaces during peak heat.
- Ask a doctor about salt replacement if you take medicines affecting fluid balance.
- Act early on cramps or exhaustion, as core temperature can rise faster.
- Avoid strenuous activity in the hottest part of the day.
Sources
Being underweight
Moderate risklimited evidenceWhy heat is harder
- Lower fat and fluid reserves mean less buffer against fluid and salt shifts.
- Low body weight often goes with muscle loss and undernutrition, which reduce the ability to cope with heat.
- In older people it commonly signals frailty, and most of the risk appears to run through frailty rather than low weight itself.
What helps
- Drink regularly even without feeling thirsty.
- Keep eating despite reduced appetite in heat.
- Arrange a daily check-in if you live alone.
- Watch for dizziness or confusion as early warning signs.
Sources
Sickle cell disease
Higher riskstrong evidenceWhy heat is harder
- Fluid loss concentrates haemoglobin inside red cells, which makes sickling more likely.
- Dehydration narrows blood vessels, adding to the risk of a blockage.
- Acidosis from dehydration shifts oxygen release and promotes further sickling.
What helps
- Drink through the day rather than only when thirsty, and aim higher than usual in hot weather.
- Act at once on dizziness, dry mouth, dark urine or headache, as a crisis can escalate quickly.
- Avoid strenuous activity between 11am and 3pm.
- Avoid alcohol, which speeds up dehydration.
Sources
Anaemia
Moderate riskmoderate evidenceWhy heat is harder
- Less haemoglobin means the heart works harder to deliver enough oxygen, even at rest.
- In heat, blood is also diverted to the skin for cooling, competing with oxygen delivery.
- Resting temperature control holds up reasonably well in studies, so the clearer risk is heat plus exertion.
What helps
- Avoid strenuous activity in peak heat.
- Rest in cool, shaded or air conditioned spaces.
- Get worsening palpitations, breathlessness or dizziness checked.
- Wear loose, breathable clothing.
Sources
Cystic fibrosis
Higher riskstrong evidenceWhy heat is harder
- The CFTR defect stops salt being reabsorbed in sweat ducts, so sodium and chloride losses are several times normal.
- Salt and water are lost together, so blood sodium does not rise and thirst is never triggered. Dehydration can progress without you feeling thirsty.
- Low blood sodium also thickens mucus, worsening chest symptoms.
What helps
- Increase salt through food or supplements in hot weather, as advised by your CF team.
- Use drinks containing sodium during heat or exercise rather than water alone.
- Drink regularly rather than waiting for thirst.
- Watch for nausea, headache, cramps or weakness, which can indicate low sodium.
Sources
Gout
Moderate riskmoderate evidenceWhy heat is harder
- Dehydration reduces urine flow, and the kidneys need that flow to clear uric acid.
- More concentrated blood makes it easier for uric acid crystals to form in joints.
- Those crystals set off the inflammation of an acute flare, and UK data shows a summer peak.
What helps
- Drink water proactively, as higher fluid intake is linked to fewer flares.
- Keep taking your usual urate lowering medicine.
- Avoid alcohol, which both triggers gout and dehydrates.
- Get early joint pain and swelling treated promptly.
Sources
IBD and ostomy
Higher riskmoderate evidenceWhy heat is harder
- Sweating adds to the fluid and sodium already lost through diarrhoea in active disease.
- A stoma, especially an ileostomy, bypasses the colon where most water and sodium are reabsorbed.
- High output stomas lose water, sodium and magnesium together, and hot weather increases output.
What helps
- Use oral rehydration solutions or isotonic drinks rather than plain water alone.
- Add extra salt to food on hot days to replace sodium lost through sweat and stoma output.
- Contact your IBD team or stoma nurse if output stays high and watery or symptoms flare.
- Check urine colour and watch for dizziness, cramps or dry mouth.
Sources
Coeliac disease
Lower risklimited evidenceWhy heat is harder
- No authoritative source links heat exposure to coeliac disease severity once the condition is managed.
- If the condition is poorly controlled with ongoing diarrhoea, the general dehydration risk of any diarrhoea applies.
- This is reasoning from general principles rather than coeliac specific evidence.
What helps
- If your coeliac disease is not well controlled and you have diarrhoea, treat it as a dehydration risk and increase fluids.
- Raise persistent symptoms with your team rather than assuming heat is the cause.
- Watch urine colour as a simple check.
Sources
Multiple sclerosis
Moderate riskstrong evidenceWhy heat is harder
- A small rise in core temperature slows conduction along demyelinated nerves, temporarily worsening symptoms. This is Uhthoff's phenomenon.
- Fatigue, blurred vision, weakness, balance and concentration are most often affected, and around 6 in 10 people with MS report heat sensitivity.
- The effect is temporary and does not mean new nerve damage or a relapse.
What helps
- Cool down actively with cool showers, cold water on wrists and face, or cooling vests and neck wraps.
- Choose lower intensity activity, exercise in cooler hours, and pre-cool beforehand.
- Contact your GP if symptoms are unusual, last longer than normal, or come with signs of infection.
- Eat cold foods with a high water content and keep cold drinks handy.
Sources
Parkinson's disease
Higher riskstrong evidenceWhy heat is harder
- Damage in the hypothalamus and brainstem disrupts control of sweating and skin blood flow.
- Autonomic problems cause abnormal sweating, either too much or too little, and poor heat loss.
- Anticholinergic medicines reduce sweating, and dopaminergic medicines can worsen blood pressure drops on standing.
What helps
- Ask a doctor or pharmacist to review medicines before a heatwave.
- Watch for dizziness rather than relying on thirst, and drink regularly.
- Use fans and cool cloths, and move to cool rooms in peak heat.
- Avoid exertion in the hottest hours and rest more.
Sources
Epilepsy
Moderate riskmoderate evidenceWhy heat is harder
- Dehydration and sweating shift sodium and glucose, which can lower the seizure threshold.
- Heat disrupts sleep, and sleep loss is a well established seizure trigger.
- Topiramate and zonisamide reduce sweating, and carbamazepine and oxcarbazepine can lower blood sodium.
What helps
- Check whether your medicine reduces sweating or affects sodium, and get bloods checked if you feel unwell.
- Use rehydration salts if sweating heavily rather than water alone.
- Protect your usual sleep routine during hot nights.
- Make sure people around you know your seizure first aid plan.
Sources
Migraine
Moderate riskmoderate evidenceWhy heat is harder
- Heat causes mild dehydration, which reduces blood volume and is a known trigger.
- Bright sunlight and glare activate light sensitive migraine pathways.
- Hot nights disrupt sleep, another common trigger.
What helps
- Carry your rescue medication when out in the heat.
- Use sunglasses, a hat and shade, and avoid peak UV hours.
- Keep sleep as regular as you can with a fan and light bedding.
- Use a cool compress on the head or neck if an attack starts.
Sources
Spinal cord injury
Higher riskstrong evidenceWhy heat is harder
- Sweating and skin blood flow are disrupted below the level of the injury, so cooling is only partial.
- The higher the injury, the more sympathetic control is lost and the less the body can sweat. Injuries above the sixth thoracic vertebra carry the greatest risk.
- Core temperature can drift with the environment rather than being held steady.
What helps
- Use external cooling actively: misting spray, damp cloths, cooling vests and fans.
- Check skin regularly, since reduced sensation means hot surfaces and sunburn can cause injury unnoticed.
- Drink to a schedule rather than to thirst.
- Plan trips around access to cool spaces and stop activity early if you feel unwell.
Sources
Quadriplegia and paraplegia
Higher riskstrong evidenceWhy heat is harder
- People with tetraplegia often have absent or greatly reduced sweating over the whole body.
- Loss of vasomotor control means blood flow cannot be redirected to the skin to shed heat.
- Paraplegia preserves more sweating capacity than tetraplegia, so risk tracks the level of injury. Reduced mobility also prevents moving somewhere cooler.
What helps
- Treat external cooling as essential rather than optional: fans, sprays and cooling garments.
- Set drinking reminders, as thirst is an unreliable guide.
- Have someone check on you at least daily during a heatwave.
- Get help quickly for confusion, headache or feeling very unwell in heat.
Sources
Cerebral palsy
Higher riskmoderate evidenceWhy heat is harder
- Most children studied have some difficulty regulating body temperature in hot and cold conditions.
- Spasticity raises energy use and heat production, while movement patterns limit heat loss.
- Reduced mobility can prevent moving somewhere cooler or fetching a drink.
What helps
- Carers should check room temperature, shade and ventilation rather than waiting to be asked.
- Offer fluids on a regular schedule.
- Use cooling aids such as damp cloths and cooling vests, especially around physiotherapy.
- Treat increased spasticity or dystonia as a possible sign of heat discomfort.
Sources
Muscular dystrophy
Higher riskmoderate evidenceWhy heat is harder
- Weak respiratory muscles make breathing harder in hot, humid air.
- Immobility prevents moving to a cooler place or adjusting clothing and bedding.
- Reduced muscle mass and mobility limit normal heat loss responses.
What helps
- Keep one room cool and well ventilated as a base during hot spells.
- Set a carer led schedule for drinks and repositioning.
- Watch for worsening breathlessness and have a plan for who to call.
- Check that any ventilator or breathing equipment is within its temperature limits.
Sources
Motor neurone disease
Higher riskmoderate evidenceWhy heat is harder
- Respiratory muscle weakness is present in most people at diagnosis, making hot humid air harder to breathe.
- Immobility prevents moving to cooler areas without help.
- Swallowing difficulty can reduce fluid intake just when more is needed.
What helps
- Prioritise a cool, ventilated room, particularly where breathing support is used.
- Offer thickened fluids or ice chips if swallowing is difficult, on advice from the speech and language team.
- Monitor breathing closely in heat and agree an escalation plan.
- Check equipment tolerance to heat.
Sources
Dementia
Higher riskstrong evidenceWhy heat is harder
- Brain changes in dementia reduce recognition of thirst, and ageing sweat glands produce less sweat.
- Memory problems mean drinks are forgotten even when offered, and layers may not be managed or a heat source moved away from.
- Antipsychotics prescribed for behavioural symptoms impair temperature control, and walking about adds outdoor exposure.
What helps
- Offer drinks often through the day and leave water visible within reach.
- Ask the GP to review antipsychotics and other medicines before summer.
- Choose light, loose, natural fibre clothing and check it is still suitable as the day warms.
- Keep an eye on walking about and make sure cool areas are safe and accessible.
Sources
Learning disability
Higher riskmoderate evidenceWhy heat is harder
- With a learning disability, understanding or reporting internal symptoms can be difficult, so distress has to be read by others.
- Hydration, clothing and room temperature often depend on carers.
- Higher rates of epilepsy, obesity and heart conditions add further heat vulnerability.
What helps
- Watch for non-verbal signs of overheating such as agitation, flushed skin or a fast pulse.
- Ask the GP or practice nurse in advance about medicines that raise heat risk.
- Carers should offer water and check clothing proactively.
- Use a simple visual routine to reduce anxiety when plans change because of heat.
Sources
Autism
Moderate risklimited evidenceWhy heat is harder
- Many autistic people register internal signals less clearly, so thirst and overheating may arrive late or not at all.
- Sensory differences can make the cooling itself unbearable. Sweat, sunscreen, hats and cold water can be painful, not just unpleasant.
- Masking hides heat distress, so people around you may not see you are struggling.
What helps
- Do not wait to feel thirsty. Put drinks on a timer or tie them to things you already do.
- Find the cooling you can tolerate before a hot spell, and keep it to hand. A damp flannel, a foot bath or a cool shower all work, so pick the texture you do not hate.
- Put a thermometer in your bedroom and living room. A number you can read beats working out how hot you feel.
- Treat rising agitation, shutdown or overload in hot weather as a possible heat signal, and check your temperature and fluids.
Sources
ADHD
Moderate risklimited evidenceWhy heat is harder
- Drinking water is a small repeated task, and ADHD makes exactly that kind of task easy to drop. A hot day can pass with almost no fluid.
- Hyperfocus can keep you outdoors, or in a hot room, for hours past the point where you should have stopped and cooled down.
- ADHD stimulants raise heat production, narrow small blood vessels and blunt thirst, at rest as well as during exercise.
What helps
- Set repeating alarms to drink, and keep a bottle where you will trip over it. Do not rely on noticing thirst.
- Set an alarm before you start anything absorbing outdoors, so hyperfocus does not decide when you stop.
- Ask your prescriber about hot weather if you have overheated before, or if you exercise or work outdoors. Do not stop or reduce ADHD medication on your own.
- Protect your sleep in a hot spell. A few bad nights will make your symptoms worse.
Sources
Schizophrenia and severe mental illness
Higher riskstrong evidenceWhy heat is harder
- Antipsychotics disrupt temperature control, partly by blunting the signals that increase skin blood flow.
- Most antipsychotics were linked to raised death rates during an extreme heat event, particularly haloperidol, clozapine and zuclopenthixol.
- Reduced help seeking and social isolation delay recognition of heat illness, and taking several psychiatric medicines together raises risk further.
What helps
- Ask for a medication review before summer, and never stop antipsychotics on your own.
- If you take clozapine, discuss hot weather plans with the clozapine clinic.
- Arrange proactive welfare checks during a heatwave rather than waiting for someone to ask for help.
- Treat new confusion as a possible medical emergency, not only a mental health symptom.
Sources
Depression
Moderate riskmoderate evidenceWhy heat is harder
- SSRIs and SNRIs affect temperature control and can cause heavy sweating, adding to fluid loss.
- Tricyclic antidepressants reduce sweating through anticholinergic effects, impairing the main cooling route.
- The risk of serotonin syndrome, which raises body temperature, rises with heat and with several serotonergic drugs together.
What helps
- Find out whether your antidepressant increases sweating or reduces it, as the risks differ.
- Do not add other serotonergic medicines or supplements without advice, particularly in hot weather.
- Use simple reminders for drinking and cooling if motivation is low.
- Ask someone to check in on you during a heatwave.
Sources
Anxiety
Moderate risklimited evidenceWhy heat is harder
- Anxiety symptoms such as a racing heart, sweating and breathlessness overlap with early heat illness, so one can be mistaken for the other.
- Heat and poor sleep can worsen anxiety directly.
- Some anxiety medicines affect alertness, which slows recognition of overheating.
What helps
- If symptoms flare in heat, check hydration and temperature before assuming it is only anxiety.
- Use grounding techniques somewhere cool rather than staying in a hot room.
- Keep a regular sleep routine with a fan and light bedding.
- Get medical help if confusion, vomiting or a very high temperature develop.
Sources
Previous heatstroke
Higher riskmoderate evidenceWhy heat is harder
- Heat intolerance can persist after heatstroke, linked to altered heat shock protein response and immune changes.
- Around a quarter of people have lasting neurological effects that affect future coping with heat.
- Long term cardiovascular risk is raised after heatstroke, adding further vulnerability.
What helps
- Discuss return to exercise or hot work with a doctor, and build back up gradually rather than alone.
- Set a lower threshold than before for stopping and cooling down.
- Have cardiovascular risk factors checked.
- Avoid alcohol before heat exposure.
Sources
Malignant hyperthermia susceptibility
Moderate riskmoderate evidenceWhy heat is harder
- RYR1 gene variants underlying malignant hyperthermia are also linked to exertional heat illness and muscle breakdown.
- Both involve disturbed calcium handling in muscle, suggesting a shared vulnerability.
- RYR1 variants account for a notable share of exertional rhabdomyolysis in otherwise healthy people.
What helps
- Always tell an anaesthetist about any personal or family history before surgery.
- Avoid strenuous exercise in heat if you or a close relative has confirmed susceptibility.
- Stop exercising at once if you get unusual muscle pain, cramping or dark urine.
- Consider genetic counselling to clarify risk for relatives.
Sources
Diabetes insipidus and pituitary damage
Higher riskstrong evidenceWhy heat is harder
- Without enough vasopressin the kidneys cannot concentrate urine, so large volumes of dilute urine are lost regardless of hydration.
- Heat adds sweat losses on top of the obligatory urine losses, so dehydration and high blood sodium develop fast.
- Desmopressin controls water loss but does not remove the underlying vulnerability.
What helps
- Never skip or delay desmopressin, and discuss hot weather dosing with your endocrine team.
- Increase fluids deliberately in heat, since thirst and the kidneys cannot compensate.
- Carry a medical alert card or bracelet naming the condition and your medication.
- Watch for confusion, severe thirst, headache or a change in urine output.
Sources
Glaucoma
Lower riskstrong evidenceWhy heat is harder
- Anticholinergic drugs widen the pupil, which in narrow drainage angles can block the flow of fluid inside the eye.
- That blockage can cause a sudden rise in eye pressure and acute angle closure glaucoma, a sight threatening emergency.
- This matters in hot weather because anticholinergics are sometimes considered to reduce sweating.
What helps
- Tell any prescriber you have glaucoma before starting an anticholinergic for sweating or anything else.
- Learn the emergency signs: sudden eye pain, blurred vision, haloes around lights, a red eye and nausea. Seek urgent care.
- Ask about non-anticholinergic options for excessive sweating, such as topical treatments or botulinum toxin.
- Keep up regular eye checks if you take an anticholinergic long term.
Sources
Asthma
Moderate riskstrong evidenceWhy heat is harder
- Hot, humid air can tighten and narrow the airways, triggering wheeze, cough and breathlessness.
- Hot sunny weather raises ground level ozone, and warm weather often brings high pollen counts.
- Thunderstorms can break pollen into fragments that reach deep into the lungs, occasionally causing sudden severe attacks.
What helps
- Keep your reliever inhaler with you at all times.
- Store inhalers below 25C and out of direct sun.
- Check pollen and air quality forecasts and stay away from busy roads.
- Exercise outdoors early or late, avoiding 11am to 3pm.
Sources
COPD
Higher riskstrong evidenceWhy heat is harder
- Higher temperatures increase breathlessness, especially with activity.
- Heat and air pollution rise together, and each independently raises the risk of a flare up.
- English NHS data shows COPD admissions rising with average summer temperature, and heatwaves bring more pneumonia admissions.
What helps
- Keep rescue medication to hand and check the air quality forecast before going out.
- Cut back outdoor activity during peak heat and high pollution hours.
- Contact your GP or respiratory team early if breathing worsens rather than waiting.
- Use a fan for breathlessness.
Sources
Sleep apnoea
Moderate riskmoderate evidenceWhy heat is harder
- Hot nights make sleep lighter and more broken, worsening apnoea related waking.
- People sometimes stop using CPAP in hot weather because the mask feels uncomfortable.
- Poor sleep and heat both reduce daytime tolerance of further heat.
What helps
- Cool the bedroom with a fan and blackout blinds rather than abandoning CPAP.
- Clean the mask and check the fit if sweating makes it slip.
- Ask for a review if daytime sleepiness worsens noticeably in hot spells.
- Avoid alcohol before bed.
Sources
Eczema
Moderate riskmoderate evidenceWhy heat is harder
- Sweat contains salts and proteins that irritate an already impaired skin barrier, driving itch.
- Heat widens blood vessels and increases the sensation of itch directly.
- Blocked sweat ducts cause prickly heat, which is more likely where the barrier is already disrupted.
What helps
- Rinse or gently pat sweat away, then reapply emollient.
- Test sunscreens on a small area first and pick fragrance free mineral ones.
- Choose lightweight cotton over synthetic fabrics.
- Keep emollients somewhere cool, as they feel more soothing chilled.
Sources
Psoriasis
Lower riskmoderate evidenceWhy heat is harder
- Many people improve in summer because sunlight has an anti-inflammatory effect on plaques, but this is not universal.
- Trapped sweat under clothing or in skin folds causes irritation, and sunburn triggers new plaques at the injured site.
- Air conditioning dries the air and the skin, which can also aggravate psoriasis.
What helps
- Get sensible sun exposure but never let skin burn.
- Keep up prescribed treatments even if plaques improve in summer.
- Pat skin dry after sweating and moisturise with a fragrance free emollient.
- Use a humidifier if air conditioning dries your skin.
Sources
Rosacea
Moderate riskmoderate evidenceWhy heat is harder
- Heat directly widens facial blood vessels, causing flushing.
- People with rosacea show exaggerated nerve, blood flow and sweating responses to heat.
- Hot drinks and sun exposure add to the same flushing pathway, and surveys put heat among the most reported triggers.
What helps
- Wear a wide brimmed hat and use a mineral sunscreen daily.
- Swap hot drinks for cool or iced ones on hot days.
- Keep a cooling mist or damp cloth handy for your face.
- Note your own triggers, as they vary between people.
Sources
Hidradenitis suppurativa
Moderate riskmoderate evidenceWhy heat is harder
- Sweat mixed with skin oils blocks follicles in skin folds, a key step in lesion formation.
- Friction from skin on skin contact or tight clothing generates local heat and irritation.
- Heat and humidity favour bacterial growth in the armpits, groin and under the breasts, where heat builds.
What helps
- Wear loose, breathable fabrics rather than tight or synthetic clothing.
- Pat or blow dry moisture prone areas rather than rubbing them.
- Use a fragrance free antiperspirant on affected folds if you tolerate it.
- Change promptly out of wet swimwear or sweaty gym clothes.
Sources
Heat and cholinergic urticaria
Lower riskmoderate evidenceWhy heat is harder
- Cholinergic urticaria is triggered by a rise in core temperature from exercise, hot baths, fever or spicy food, releasing histamine from mast cells.
- Small, intensely itchy weals appear within five to ten minutes and usually settle within an hour or two.
- Heat urticaria is a separate, rarer reaction to local warmth on the skin.
What helps
- Take cool showers rather than hot ones and avoid saunas and hot tubs.
- Ask about taking a non-sedating antihistamine before known triggers.
- Warm up and cool down gradually around exercise rather than sudden heat.
- Get urgent help if you ever get wheeze, swelling or faintness with a reaction.
Sources
Hyperhidrosis
Lower riskstrong evidenceWhy heat is harder
- Sweating goes well beyond what temperature control needs, and heat, stress and exercise increase it further.
- Heavy sweat loss in hot weather adds to dehydration risk.
- Constant moisture irritates and softens the skin, raising infection risk.
What helps
- Use a prescribed aluminium chloride antiperspirant, applied to dry skin at night.
- Ask your GP about further options such as iontophoresis or botulinum toxin injections.
- Wear breathable, moisture wicking fabrics and carry a change of clothes.
- Dry skin folds carefully to reduce infection risk.
Sources
Anhidrosis and hypohidrotic ectodermal dysplasia
Higher riskstrong evidenceWhy heat is harder
- Sweat glands are reduced or absent, so the body cannot cool itself by evaporation.
- Sweating is the main human cooling mechanism, so even moderate heat or exercise can be dangerous.
- Core temperature can rise to a level that risks seizures, brain injury or death without active cooling.
What helps
- Use external cooling actively: damp cloths, cooling vests, misting bottles and battery fans.
- Wet clothing deliberately to mimic evaporation when needed.
- Carry cold drinks and ice and plan activities around where cooling is available.
- Get urgent help for confusion, very hot dry skin or collapse in heat.
Sources
Sympathectomy for hyperhidrosis
Moderate riskmoderate evidenceWhy heat is harder
- The operation is very effective for hand and armpit sweating but commonly causes compensatory sweating elsewhere on the trunk.
- Most patients get some compensatory sweating and a substantial minority find it severe.
- Whether this is a normal cooling reflex or a consequence of nerve damage is debated, so it may not help with cooling.
What helps
- Discuss the risk of compensatory sweating fully with a surgeon before choosing this operation.
- If it develops, use breathable clothing and antiperspirants on the newly affected areas.
- Revisit non-surgical options if compensatory sweating is severe.
- Keep fluids up, as sweat losses may still be high overall.
Sources
Mast cell activation syndrome and histamine intolerance
Moderate risklimited evidenceWhy heat is harder
- Heat is widely reported as a trigger for mast cells releasing histamine, alongside cold, friction, exertion and stress.
- Histamine widens blood vessels, and heat does the same, so flushing is more pronounced and heavy release can cause low blood pressure or collapse.
- Evidence specifically isolating heat as a trigger is thin, and much of it is patient reported.
What helps
- Carry your prescribed antihistamines and any emergency adrenaline pen when out in heat.
- Keep a trigger diary, as heat sensitivity varies a lot between individuals.
- Sit or lie down at the first sign of flushing with lightheadedness.
- Get urgent help for breathing difficulty or collapse.
Sources
Lupus and photosensitive autoimmune disease
Higher riskstrong evidenceWhy heat is harder
- UV light triggers or worsens lupus in a large proportion of patients.
- Photosensitivity can cause skin rashes and can also set off whole body flares with joint pain and fatigue.
- Reactions can appear within minutes or take hours to days, so the sun link is easy to miss.
What helps
- Use broad spectrum high factor sunscreen daily, including on cloudy days.
- Wear sun protective clothing and a wide brimmed hat.
- Keep taking hydroxychloroquine as prescribed. It gives some photoprotection but is not a substitute for sunscreen.
- Report new rashes or flares rather than assuming it is only sunburn.
Sources
Scleroderma
Moderate riskmoderate evidenceWhy heat is harder
- Skin thickening and scarring can reduce the ability to sweat, impairing heat loss.
- Raynaud's affects almost everyone with systemic sclerosis, and symptoms can worsen at high outdoor temperatures, probably through air conditioning exposure.
- So sweating is impaired in heat while rapid cooling provokes vascular symptoms. Moving between hot air and cold air is the problem.
What helps
- Carry a light layer for moving between hot outdoors and air conditioned indoors.
- Avoid very cold air conditioning settings and aim for a gentler difference.
- Take extra care with hydration, since sweating may be impaired.
- Warm hands gradually rather than under very hot water.
Sources
Raynaud's phenomenon
Lower riskmoderate evidenceWhy heat is harder
- Raynaud's is fundamentally a cold triggered condition, and cold remains the main risk. Heat itself is not a direct trigger.
- Sudden temperature drops, such as walking from hot air into strong air conditioning, can also provoke an attack.
- Research in systemic sclerosis found symptom severity rising at very high outdoor temperatures, plausibly through air conditioning use.
What helps
- Keep a light layer to hand when moving between hot outdoors and cold indoor spaces.
- Avoid very low air conditioning settings and use fans instead where possible.
- Warm hands and feet gradually rather than under very hot water.
- Report new ulcers or skin changes on fingers or toes.
Sources
Rheumatoid arthritis
Moderate risklimited evidenceWhy heat is harder
- Biologic and other immunosuppressive treatments raise the risk of serious infection.
- Biologic injections need cold chain storage, usually 2 to 8C, and heat degrades them.
- High humidity may increase tissue swelling around joints, and heat and fatigue interact so activity tolerance can fall.
What helps
- Keep biologic injections refrigerated and use a proper cool bag when travelling, never a hot car.
- Take signs of infection seriously rather than assuming tiredness is just the heat.
- Use cool packs or cool showers if joints feel stiffer in humid weather.
- Pace activity across the cooler parts of the day.
Sources
Cancer and cancer treatment
Higher riskstrong evidenceWhy heat is harder
- Treatment side effects such as diarrhoea, vomiting and poor appetite impair hydration and temperature control.
- Some chemotherapy drugs and radiotherapy make skin sun sensitive, and treated skin stays that way for months afterwards.
- Heat worsens cancer related fatigue, and lymphoedema limbs swell more and are prone to infection.
What helps
- Protect treated skin with clothing and shade, not sunscreen alone.
- With lymphoedema, avoid prolonged heat on the affected limb and keep wearing compression as advised.
- Contact your care team early if you feel unwell during a heatwave.
- Watch for dizziness, confusion or passing less urine.
Sources
Immunosuppression
Higher riskmoderate evidenceWhy heat is harder
- Many immunosuppressant medicines need cold storage, and heat degrades them or allows contamination.
- Fever, a key warning sign of infection, can be masked or blunted by some treatments.
- Reduced ability to fight infection makes skin breaks, insect bites and summer water exposure riskier than usual.
What helps
- Store injectable medicines in the fridge and use an insulated cool bag when travelling.
- Seek advice promptly for fever or feeling generally unwell rather than waiting.
- Clean and watch any cuts or bites carefully.
- Avoid stagnant water and unclean pools.
Sources
HIV
Lower risklimited evidenceWhy heat is harder
- Antiretroviral medicines are generally stable at room temperature but should still be kept out of direct heat.
- Heart and metabolic conditions that often occur alongside HIV interact with heat stress.
- Well controlled HIV on current treatment carries much lower risk than in the past.
What helps
- Store medication as the label says and keep it out of hot cars and direct sun.
- Keep up regular clinical review, since good control reduces heat vulnerability.
- Watch for heat symptoms, especially if you also have heart or metabolic conditions.
- Take skin problems and infections seriously in hot weather.
Sources
Fibromyalgia
Moderate riskmoderate evidenceWhy heat is harder
- Central sensitisation means an amplified pain response, and most studies find a lowered pain threshold to heat specifically.
- Heat exposure in research settings increased headache, pain, anxiety, fatigue and low mood.
- Sensitivity runs both ways, so cold can also worsen symptoms, and individual variation is wide.
What helps
- Plan activity for cooler parts of the day and pace yourself.
- Use fans, cool cloths and cool showers before symptoms build rather than after.
- Track your own temperature triggers, since these vary between people.
- Protect sleep during hot nights.
Sources
ME/CFS and long COVID
Moderate risklimited evidenceWhy heat is harder
- Both often involve autonomic dysfunction and orthostatic intolerance, which heat worsens by widening blood vessels and pooling blood in the legs.
- Post-exertional malaise can be provoked by the added physiological stress of heat.
- Temperature dysregulation, including hot flushes, chills and sweating, is itself a recognised symptom.
What helps
- Avoid exertion in peak heat and rest before you feel you need to.
- Sit or lie down if you feel lightheaded, rather than pushing on.
- Discuss extra salt or electrolytes with a clinician.
- Plan a lighter schedule during a heatwave to reduce the risk of a crash.
Sources
Hypermobility and Ehlers-Danlos syndromes
Moderate riskmoderate evidenceWhy heat is harder
- Most people with hypermobility or EDS also have POTS, linked to more stretchy blood vessels that let blood pool on standing.
- Heat widens blood vessels further, adding to pooling and worsening dizziness and palpitations.
- Temperature dysregulation and heavy sweating are recognised symptoms, and dehydration, prolonged standing and large meals compound the effect.
What helps
- Increase fluids and, where clinically advised, salt intake.
- Avoid prolonged standing in hot conditions and use compression garments if recommended.
- Rise slowly from sitting or lying to reduce dizziness.
- Sit down at the first sign of feeling faint.
Sources
POTS and dysautonomia
Higher riskmoderate evidenceWhy heat is harder
- Heat widens blood vessels, which drops blood pressure and pushes heart rate up further.
- Blood pools in the legs on standing, so POTS symptoms worsen when upright in heat.
- Even mild dehydration reduces blood volume and makes orthostatic intolerance more pronounced.
What helps
- Increase fluids and, where advised, salt to help maintain blood volume.
- Use compression stockings or an abdominal binder if these have been recommended.
- Avoid prolonged standing and move activities to early morning or evening.
- Sit or lie with legs raised at the first sign of feeling faint.
Sources
Pregnancy
Higher riskmoderate evidenceWhy heat is harder
- Pregnancy raises metabolic rate and blood volume, increasing baseline heat load and leaving less reserve to shed extra heat.
- Heat can reduce blood flow between the placenta and baby, linked to babies being small for their age.
- The odds of preterm birth rise with temperature, and early pregnancy heat exposure is linked to some congenital anomalies.
What helps
- Contact your midwife, GP or 111 straight away for sudden swelling, a severe headache or reduced baby movements.
- Keep out of the heat between 11am and 3pm, and drink more than usual.
- Avoid strenuous activity, especially early in a heatwave.
- Rest with your feet up, take cool showers, and wear loose, light clothing.
Sources
Menopause and perimenopause
Lower riskmoderate evidenceWhy heat is harder
- Falling oestrogen narrows the body's internal thermostat range, making it oversensitive to small temperature changes.
- Ambient heat adds to this, so hot flushes and night sweats become more frequent and more intense.
- Heat driven night sweats fragment sleep on top of menopause related sleep problems.
What helps
- Speak to a GP if flushes are frequent or severe, as HRT may be worth discussing.
- Cut back alcohol and caffeine, both common flush triggers.
- Take cool showers and keep the bedroom cool overnight.
- Wear loose cotton layers you can remove easily.
Sources
Babies under 1
Higher riskstrong evidenceWhy heat is harder
- Immature sweating and a high surface area relative to body mass mean babies gain and lose heat much faster than adults.
- Babies cannot remove their own bedding or clothing, so carers must manage layers for them.
- Overheating is an established risk factor for sudden infant death syndrome, and cool hands and feet are normal rather than a reliable sign.
What helps
- Keep the sleep room between 16C and 20C, using a room thermometer, and reduce to a nappy or vest with no extra bedding in hot weather.
- Never cover a pram with a blanket, as it traps heat. Use a clip-on parasol instead.
- No sunscreen under 6 months. Use shade, clothing and a wide brimmed hat. From 6 months use SPF 30 or higher.
- Check the chest or back of the neck for overheating, and offer more frequent breastfeeds or cooled boiled water between formula feeds.
Sources
Young children under 5
Higher riskstrong evidenceWhy heat is harder
- A high surface area relative to body mass and immature sweating mean children heat up and dehydrate faster than adults.
- Young children cannot reliably recognise or communicate thirst and distress, and higher activity adds to fluid loss.
- In a parked car the inside temperature can rise around 11C within 10 minutes, and a child's core temperature rises several times faster than an adult's.
What helps
- Never leave a child alone in a parked car, even briefly, and always check the back seat before locking up.
- Offer extra fluids often, including frozen water lollies for toddlers.
- Keep bedrooms between 16C and 20C with light bedclothes.
- Use loose light clothing, a wide brimmed hat and SPF 30 or higher from 6 months.
Sources
Older age 65 and over
Higher riskstrong evidenceWhy heat is harder
- Reduced thirst sensation means older people drink less than they need without feeling thirsty.
- Reduced and delayed sweating impairs cooling, and reduced cardiac reserve limits blood flow to the skin.
- Polypharmacy adds risk, as diuretics dehydrate and anticholinergics and beta blockers interfere with sweating.
What helps
- Ask a GP or pharmacist to review medicines ahead of hot spells.
- Check room temperature regularly and keep living spaces cool with curtains closed in the sun.
- Drink small amounts of water often rather than waiting for thirst.
- Keep in regular contact with someone, and seek help for confusion, dark urine or dizziness.
Sources
Older age 75 and over
Higher riskstrong evidenceWhy heat is harder
- All the mechanisms affecting people over 65 apply, more severely.
- Heatwave mortality risk rises more steeply in this group than in those aged 65 to 74.
- Frailty, multiple long term conditions and care dependence compound heat risk.
What helps
- Arrange daily contact during amber or red heat alerts.
- Keep the coolest room in the home prepared as a base before the heat builds.
- Check medicines are stored correctly, as some need refrigeration in extreme heat.
- Agree in advance who to call and when to call 111 or 999.
Sources
Reduced mobility
Higher riskmoderate evidenceWhy heat is harder
- With reduced mobility people cannot move themselves to shade or a cooler room, or open a window unaided, so heat exposure continues.
- Hydration depends on carer visits rather than being able to fetch a drink.
- Heat and sweat soften the skin and raise the risk of pressure sores at contact points.
What helps
- Build heat into care plans: hydration, room temperature, clothing and medicine storage as explicit items.
- Increase check-in frequency during heat alerts.
- Offer fluids proactively and leave drinks within reach between visits.
- Check skin at pressure points more often and keep bedding breathable and dry.
Sources
Wheelchair use
Higher risklimited evidenceWhy heat is harder
- Metal frames and footplates on a wheelchair left in the sun heat quickly and can burn skin, especially where sensation is reduced.
- Sitting reduces air movement around the torso and thighs, and cushions and non-breathable covers trap heat and moisture.
- Where spinal cord injury impairs sweating and blood flow control, the cooling response is directly compromised.
What helps
- Check metal parts for heat before touching them and keep the chair shaded when parked in sun.
- Use a cooling neck wrap, damp cloth or ice pack on the neck and wrists.
- Check skin at seat contact points more often in hot weather.
- Act early on dizziness, nausea or cramps rather than pressing on.
Sources
Deafness and blindness
Moderate risklimited evidenceWhy heat is harder
- Heat health alerts go out mainly via TV, radio and text, formats that are partly or wholly inaccessible without captions, audio description or accessible text.
- Visual signs of heat illness such as flushed skin are harder to self-assess or spot in a companion without sight.
- Navigating to unfamiliar cool public spaces is harder with sight loss.
What helps
- Register for emergency SMS by texting register to 999, and set up Relay UK in advance.
- Ask a family member, carer or neighbour to relay heat alert content in an accessible format.
- Have a known contact accompany a first visit to a recommended cool public space.
- Keep a device charged in case power cuts disrupt screen readers or captioned TV.
Sources
Homelessness and rough sleeping
Higher riskmoderate evidenceWhy heat is harder
- No shade or shelter, with sleeping on hot ground or in poorly ventilated tents, worsened by the urban heat island effect.
- No reliable access to water, or to safe cool storage for medicines such as insulin.
- Higher rates of heart, lung, kidney and mental health conditions, plus substance use, all impair heat regulation.
What helps
- Outreach teams should carry and hand out water, sunscreen, hats and light clothing during heat alerts.
- Extend day centre and hostel opening so people can access shade, water and toilets in peak heat.
- Map and signpost local cool spaces and free water refill points.
- Train staff to spot heat exhaustion and heatstroke with a clear route to emergency care.
Sources
Living alone and social isolation
Higher riskstrong evidenceWhy heat is harder
- Nobody is present to notice confusion, collapse or worsening heat illness, so discovery is delayed.
- Evidence from the 2003 French heatwave showed social isolation was a dominant risk factor for death at home.
- Isolation matters more than living alone by itself, though the two often go together.
What helps
- Arrange at least daily contact for anyone in a higher risk group who lives alone.
- Check on neighbours, friends or relatives who live alone during hot spells.
- Agree a simple daily signal such as a text, call or opened curtains during a heat alert.
- Keep emergency contact details visible in the home in case a welfare check is needed.
Sources
Occupational heat exposure
Higher riskstrong evidenceWhy heat is harder
- Occupational heat stress depends on air temperature, humidity, work rate and clothing together, not air temperature alone.
- Indoor hot work such as bakeries, foundries and kitchens can be hazardous year round.
- Protective clothing restricts sweat evaporation, and unacclimatised workers are at higher risk, including those back from leave.
What helps
- Employers should risk assess on work rate, humidity and clothing, since there is no legal maximum workplace temperature in the UK.
- Provide cool drinking water, shaded or cooled rest areas and more frequent breaks.
- Allow a phased period for new or returning workers to acclimatise.
- Train staff and supervisors to recognise heat stress with a clear response plan.
Sources
Night shift work
Moderate riskmoderate evidenceWhy heat is harder
- Sleeping during the day after shift work runs against the body clock, when core temperature is higher and less able to fall as sleep needs.
- Daytime bedrooms overheat further in summer, and each 1C rise in night temperature costs roughly 5 to 10 minutes of sleep.
- Sleep debt across a run of shifts impairs judgement and awareness of heat risk.
What helps
- Use blackout blinds plus a fan to keep the daytime sleep space cool and dark.
- Sleep in the coolest room in the house during heatwaves, even if it is not the usual bedroom.
- Employers should adjust shift timing or workload and allow more rest breaks where practical.
- Track cumulative tiredness across a run of shifts rather than assuming you are used to it.
Sources
Incontinence and fluid restriction
Higher riskmoderate evidenceWhy heat is harder
- People with incontinence often cut fluids to reduce urgency, but concentrated urine irritates the bladder and makes urgency worse.
- Reduced thirst sensation in older age adds to an existing deficit when fluids are also being restricted.
- Dehydration leads quickly to confusion, falls, urine infections and kidney strain, and heat speeds the path to kidney injury.
What helps
- Do not restrict fluids to manage incontinence. Aim for the usual 6 to 8 cups a day, more in heat, unless a clinician has advised otherwise.
- Plan toilet access with a commode, easy clothing or timed prompts so fear of accidents does not drive fluid avoidance.
- Offer cool drinks, ice lollies or foods with a high water content as other routes to fluid.
- Monitor urine colour, output and alertness more closely during hot spells.
Sources
Eating disorders
Higher riskmoderate evidenceWhy heat is harder
- Low body mass reduces the reserve available for temperature regulation, and baseline core temperature can already run lower.
- Purging through vomiting, laxatives or diuretics causes dehydration and electrolyte disturbance, both worsened by heat.
- Heart complications reduce the capacity to increase skin blood flow, and compulsive exercise in heat raises the risk of collapse.
What helps
- Use a hydration plan agreed with a clinician rather than restricting fluids out of anxiety.
- Keep to regular meals and fluids even if heat disrupts appetite or routine.
- Supporters should treat dizziness, palpitations or confusion as possible cardiac or electrolyte problems, not just heat discomfort.
- Seek medical advice early during a heatwave if purging or restriction has increased.
Sources
Alcohol dependence
Higher riskmoderate evidenceWhy heat is harder
- Alcohol is a diuretic, increasing fluid loss on top of sweating.
- Intoxication impairs judgement about heat risk, spotting warning signs and moderating activity.
- Withdrawal causes a raised heart rate, blood pressure and temperature with heavy sweating, layering onto heat stress.
What helps
- Seek medical advice before or during a heatwave if you are in withdrawal or cutting down, as symptoms can mimic heat illness.
- If drinking, alternate with water and add salty snacks if sweating heavily.
- Stop drinking earlier in the evening and switch to water to protect sleep.
- Ask someone to check on you during a heat alert.
Sources
Substance use
Higher riskmoderate evidenceWhy heat is harder
- Stimulants such as MDMA, cocaine and amphetamines raise the body's temperature set point and reduce the ability to shed heat.
- Combined with dancing in hot crowded settings this can cause dangerous hyperthermia, with fatal cases recording very high core temperatures.
- MDMA also affects fluid regulating hormones, so drinking large amounts of plain water can cause dangerously low sodium.
What helps
- Take regular breaks from dancing or activity to cool down in shaded or air conditioned areas.
- Sip water steadily rather than drinking large amounts at once, to avoid low sodium.
- Start lower than usual, as effects are less predictable in high temperatures.
- Check on friends who may not notice they are overheating. Sedation from opioids can look like heatstroke, so treat an unrousable person as an emergency.
Sources
SSRIs (selective serotonin reuptake inhibitors)
Moderate riskmoderate evidenceWhy heat is harder
- Sweating more than usual is common, which increases fluid and salt loss in the heat.
- These medicines act on serotonin, which the brain uses to help set body temperature.
- Passing more urine adds to dehydration, and the circulation is slower to compensate when heat lowers blood pressure.
What helps
- Drink water regularly through the day, as sweating more raises the risk of dehydration.
- Watch for headache, dizziness or feeling faint, which can be early signs of heat exhaustion.
- Very rarely, high temperatures plus serotonin drugs can contribute to serotonin syndrome, so get urgent help for agitation, shivering, muscle twitching and a high temperature.
- Do not stop or change the dose without speaking to a prescriber first.
Sources
SNRIs (serotonin and noradrenaline reuptake inhibitors)
Moderate riskmoderate evidenceWhy heat is harder
- Sweating more than usual is common, which increases fluid and salt loss in the heat.
- These medicines act on serotonin and noradrenaline, both used by the brain to set body temperature.
- Noradrenaline effects raise the heat the body makes at rest, and passing more urine adds to dehydration.
What helps
- Excess sweating is common with these medicines, so increase fluids on hot days.
- Keep an eye on blood pressure symptoms such as dizziness on standing.
- Seek urgent help for a high temperature with agitation, tremor or muscle stiffness.
- Do not change the dose yourself, as stopping venlafaxine abruptly causes withdrawal effects.
Sources
Tricyclic antidepressants
Higher riskmoderate evidenceWhy heat is harder
- Strong anticholinergic effects mean reduced sweating, the main way the body sheds heat.
- They also interfere with the part of the brain that sets body temperature.
- Thirst is blunted, so dehydration builds up without you noticing, and the circulation compensates poorly when heat drops blood pressure.
What helps
- These have strong anticholinergic effects that cut sweating, so heat builds up more easily.
- Drink regularly even without feeling thirsty, as thirst signals can be blunted.
- Confusion, a hot dry skin or a very high temperature needs urgent medical attention.
- Keep to shade and cooler rooms during the hottest part of the day.
Sources
MAOIs (monoamine oxidase inhibitors)
Higher risklimited evidenceWhy heat is harder
- These interfere with the part of the brain that sets body temperature, and raise the heat the body produces.
- Some also have anticholinergic effects, so sweating is reduced.
- Blood pressure control is less stable, so the circulation compensates poorly for heat.
What helps
- Combining these with other serotonin drugs raises the risk of a dangerous rise in temperature, so check any new medicine with a pharmacist.
- Avoid strenuous activity outdoors when it is very hot.
- Get urgent help for a high temperature with confusion, sweating or muscle rigidity.
Sources
Mirtazapine
Moderate risklimited evidenceWhy heat is harder
- Sedation makes it harder to notice that you are overheating and to act on it.
- It affects the brain's temperature control, and blood pressure can drop on standing in the heat.
What helps
- Sedation and drowsiness can make it harder to notice you are overheating.
- Keep the bedroom as cool as possible, since the medicine is usually taken at night.
- Rise slowly from bed or a chair, as low blood pressure and dizziness are more likely in heat.
- Ask someone to check on you during a heat alert if you live alone.
Sources
Bupropion
Moderate risklimited evidenceWhy heat is harder
- It has activating effects that raise the amount of heat the body produces.
- Sweating can increase, adding to fluid loss.
- It acts on the brain chemicals involved in setting body temperature.
What helps
- This has stimulant-like effects that can add slightly to heat production and sweating.
- Seizure risk rises with dehydration and electrolyte loss, so keep fluids up.
- Avoid hard exercise in the hottest hours.
- Report a persistent high temperature or confusion to a clinician.
Sources
Lithium
Higher riskstrong evidenceWhy heat is harder
- Sweating and dehydration concentrate lithium in the blood, so levels rise on an unchanged dose.
- Lithium itself makes some people pass more urine, which adds to fluid loss.
- At raised levels it disturbs the brain's temperature control and causes confusion.
What helps
- Keep fluid intake steady and consistent rather than drinking large amounts all at once.
- Do not make sudden changes to salt intake, as this also shifts lithium levels.
- Contact a clinician urgently for vomiting, diarrhoea, coarse tremor, unsteadiness, slurred speech or drowsiness.
- A lithium level check may be advised during a prolonged hot spell.
Sources
Typical (first generation) antipsychotics
Higher riskmoderate evidenceWhy heat is harder
- Blocking dopamine interferes with the brain's temperature set point, so heat is managed poorly.
- Many also have anticholinergic effects, so sweating is reduced.
- Thirst signals are blunted, and the circulation compensates less well when heat lowers blood pressure.
What helps
- Do not reduce or stop these medicines yourself. That is a decision for your prescribing team.
- Stay in the coolest room during the hottest hours and use fans or cool damp cloths.
- Drink regularly through the day rather than waiting until you feel thirsty.
- A high temperature with muscle stiffness, confusion or sweating is a medical emergency.
Sources
Atypical (second generation) antipsychotics
Higher riskmoderate evidenceWhy heat is harder
- These interfere with the brain's temperature set point, so the body manages heat poorly.
- Several have anticholinergic effects, so sweating is reduced.
- Thirst is blunted, and blood pressure can fall in the heat with the circulation slow to compensate.
What helps
- Risperidone, quetiapine, olanzapine and clozapine have all been linked to heat stroke cases.
- Clozapine needs particular care, as a raised temperature also needs checking for other causes.
- Seek urgent medical attention for signs of heat stroke such as confusion, a very high temperature or collapse.
- Do not stop the medicine because of hot weather without clinical advice.
Sources
ADHD stimulants
Moderate risklimited evidenceWhy heat is harder
- Stimulants raise the amount of heat the body produces.
- They narrow small blood vessels, so less heat escapes through the skin.
- Appetite and thirst are reduced, so people often start the day behind on fluid.
What helps
- Set reminders to drink water, since it is easy to forget on these medicines.
- Take extra care with sport or outdoor work in high temperatures.
- Evidence from everyday use is reassuring. The risk is mainly with heavy exertion in heat.
Sources
Atomoxetine
Lower risklimited evidenceWhy heat is harder
- Sweating can increase, which adds to fluid loss on hot days.
- Noradrenaline effects narrow small blood vessels, reducing heat loss through the skin.
- Pulse and heat production both rise slightly.
What helps
- Noradrenaline effects can cause sweating and a faster heart rate in the heat.
- Keep fluids up and pace outdoor activity on hot days.
- Mention any dizziness or palpitations during hot weather to a clinician.
Sources
Guanfacine
Moderate risklimited evidenceWhy heat is harder
- This lowers blood pressure and heart rate, so the circulation cannot compensate for heat.
- It acts on brain pathways involved in temperature and alertness.
What helps
- This lowers blood pressure and heart rate, so fainting and dizziness are more likely in heat.
- Get up slowly from sitting or lying, especially in hot weather.
- Keep fluid intake steady, as dehydration adds to the drop in blood pressure.
- Report repeated dizziness or fainting to a clinician.
Sources
Anticholinergics and antimuscarinics
Higher riskstrong evidenceWhy heat is harder
- These block sweating directly, and reduced sweating removes the body's main way of shedding heat.
- The anticholinergic effect also disturbs the brain's temperature control and can cause confusion.
- Thirst is blunted, so dehydration develops without the usual warning.
What helps
- Cool the skin with damp cloths or a cool shower, since sweating cannot do the job.
- Several anticholinergic medicines taken together add up, so ask for a medicines review.
- Hot dry skin with confusion or a very high temperature is a medical emergency.
- Stay out of direct sun and keep to cool rooms when temperatures are high.
Sources
Sedating (first generation) antihistamines
Moderate riskmoderate evidenceWhy heat is harder
- Anticholinergic effects mean reduced sweating, so heat builds up more easily.
- Drowsiness makes it harder to notice you are overheating.
What helps
- A non-sedating alternative may be a better choice in hot weather, so ask a pharmacist.
- Older antihistamines reduce sweating, and drowsiness makes overheating harder to notice.
- Avoid using these as sleep aids on very hot nights.
Sources
Non-sedating (second generation) antihistamines
Lower risklimited evidenceWhy heat is harder
- These have only weak anticholinergic effects, so any reduced sweating is slight.
What helps
- These have much weaker anticholinergic effects and are generally a better option in the heat.
- Normal hydration and shade precautions are enough for most people.
- Cetirizine causes mild drowsiness in some people, so see how it affects you before driving in heat.
Sources
Beta blockers
Higher riskstrong evidenceWhy heat is harder
- Shedding heat needs a faster heart rate, and these medicines cap that response.
- They reduce blood flow to the skin, so less heat escapes.
- Sweating is reduced a little, particularly with the non-selective types.
What helps
- These blunt the rise in heart rate and skin blood flow the body needs to shed heat.
- Rise slowly from sitting or lying, as fainting and falls are more likely.
- Non-selective types such as propranolol appear to affect temperature control more than others.
- Report dizziness, unusual tiredness or fainting to a clinician rather than stopping the tablets.
Sources
ACE inhibitors and ARBs
Higher riskstrong evidenceWhy heat is harder
- Thirst is blunted, so dehydration creeps up without you feeling it.
- They increase salt and water loss through the kidneys, and blood pressure falls more readily in heat.
- When you are dehydrated the kidneys cannot maintain filtration, so kidney injury is more likely.
What helps
- Any temporary dose change should come from a clinician. Do not adjust the dose yourself.
- Combined with dehydration they raise the risk of kidney injury, especially alongside NSAIDs and diuretics.
- Ask about a kidney function and electrolyte check during a long hot spell.
- Get up slowly to avoid dizziness.
Sources
Calcium channel blockers
Moderate riskmoderate evidenceWhy heat is harder
- These already widen blood vessels, so heat can drop blood pressure further.
- Fluid shifts into the ankles, and dehydration lowers blood pressure further still.
What helps
- Blood vessels are already widened by these medicines, so heat can drop blood pressure further.
- Expect more dizziness on standing and move slowly when getting up.
- Ankle swelling often worsens in heat, which is uncomfortable but usually not dangerous.
- Rest in cool rooms during the hottest part of the day.
Sources
Thiazide diuretics
Higher riskstrong evidenceWhy heat is harder
- These increase salt and water loss, adding to what you lose through sweating.
- Losing volume means the heart and blood vessels compensate poorly for heat, and sodium levels can fall, causing confusion, weakness and falls.
- Some cause photosensitivity, so skin burns more easily in the sun.
What helps
- Do not stop the tablets on your own, as this can destabilise blood pressure or heart failure.
- Ask about a blood test for kidney function and sodium in a long hot spell.
- Low sodium levels can cause confusion, weakness and falls, so report these symptoms.
- Some thiazides increase sun sensitivity, so use sunscreen and cover up.
Sources
Loop diuretics
Higher riskstrong evidenceWhy heat is harder
- These cause the largest fluid and salt losses of any diuretic, on top of sweating.
- Low blood volume leaves the heart unable to compensate for heat.
- Dehydration raises the risk of kidney injury and disturbed electrolytes.
What helps
- Fluid advice must be balanced against any fluid restriction for heart failure, so follow the clinical plan.
- Weigh yourself daily if advised, as this tracks fluid balance better than thirst.
- Report dizziness, cramps, much reduced urine output or sudden weakness.
- Kidney function and electrolyte monitoring is often advised during heat periods.
Sources
Spironolactone and other aldosterone antagonists
Higher riskmoderate evidenceWhy heat is harder
- These increase fluid and salt loss, adding to sweat losses.
- Dehydration raises potassium levels, which affects the heart rhythm.
- Blood pressure can fall further in the heat.
What helps
- Dehydration raises the risk of high potassium levels, which affects the heart rhythm.
- Avoid potassium supplements or salt substitutes unless advised.
- Blood tests for potassium and kidney function may be needed during a heatwave.
- Report palpitations, marked weakness or a slow pulse promptly.
Sources
Statins and fibrates
Lower risklimited evidenceWhy heat is harder
- Dehydration raises blood levels of these medicines slightly, which adds to the risk of muscle damage.
What helps
- The main concern is muscle breakdown, so report severe unexplained muscle pain or dark urine.
- Keep hydrated when exercising in heat, as heavy exertion adds to muscle strain.
- No routine dose change is needed for hot weather.
Sources
Metformin
Moderate riskmoderate evidenceWhy heat is harder
- Dehydration in the heat reduces kidney function, so metformin builds up.
- At high levels it can cause lactic acidosis, whose early signs resemble heat exhaustion.
What helps
- If dehydration reduces kidney function, metformin can build up and cause lactic acidosis.
- Seek advice if you have vomiting, diarrhoea or cannot keep fluids down.
- Sick day guidance often means pausing metformin temporarily, but only on clinical advice.
Sources
SGLT2 inhibitors
Higher riskmoderate evidenceWhy heat is harder
- These push glucose out in the urine, which takes water with it and adds to sweat losses.
- The drop in blood volume lowers blood pressure and strains the kidneys.
- They shift the body towards making ketones, so dehydration can cause ketoacidosis even at normal glucose.
What helps
- Check ketones if unwell, since ketoacidosis can occur even when blood glucose looks normal.
- Seek advice if you cannot eat or drink normally, as the medicine may need pausing.
- Watch for dizziness on standing, as these medicines add to fluid loss.
- Genital and urinary infections are more likely with heat and sweating, so keep clean and dry.
Sources
Insulin and sulfonylureas
Higher riskmoderate evidenceWhy heat is harder
- Warm skin carries more blood, so injected insulin is absorbed faster and can act harder than expected.
- Dehydration concentrates the medicine and makes glucose harder to predict.
- High glucose itself increases urine output, adding to fluid loss.
What helps
- Check blood glucose more often than usual during hot weather, as heat can speed insulin absorption.
- Sweating is a warning sign of low blood glucose that is easy to miss when it is hot anyway.
- Store insulin in a fridge or cool bag and never leave it in a car or in direct sun.
- Heat can also make glucose meters and test strips read inaccurately, so keep them cool.
Sources
GLP-1 receptor agonists
Moderate risklimited evidenceWhy heat is harder
- Nausea and vomiting cause fluid loss, and reduced appetite means people drink less.
- Appetite and thirst are both suppressed, so dehydration goes unnoticed.
- Dehydration on these medicines has been linked to kidney injury.
What helps
- Make a point of drinking fluids even when appetite and thirst are low.
- Dehydration on these medicines has been linked to kidney injury, so report much reduced urine output.
- Store pens in the fridge and use a cool bag when travelling in hot weather.
Sources
Levothyroxine and thyroid medicines
Moderate riskmoderate evidenceWhy heat is harder
- Thyroid hormone raises the body's metabolic rate and the heat it produces.
- Too high a dose causes sweating and poor heat tolerance.
What helps
- Too high a dose causes sweating and heat intolerance, so mention this at review.
- A thyroid function test may be worth checking if you feel unusually hot and shaky.
- Keep fluids up, as extra sweating means extra fluid loss.
Sources
Topiramate and zonisamide
Higher riskstrong evidenceWhy heat is harder
- These cause reduced sweating, a recognised side effect reported most often in children.
- They also increase urine output, adding to fluid loss.
- Dehydration changes blood levels and raises the risk of kidney stones and acidosis.
What helps
- Report reduced sweating or a raised temperature to the prescribing team.
- The risk is highest in children and during exercise or hot weather.
- Watch for a hot child who is not sweating, and cool them and get medical advice.
- Keep fluids up, as these medicines also increase urine output.
Sources
Other antiepileptic medicines
Moderate riskmoderate evidenceWhy heat is harder
- Dehydration changes blood levels of these medicines, which can affect seizure control.
- Carbamazepine and oxcarbazepine lower sodium levels, which heat and sweating worsen.
- Broken sleep on hot nights is itself a seizure trigger.
What helps
- Never miss doses or change them because of hot weather without advice.
- Carbamazepine and oxcarbazepine can lower sodium levels, which heat and sweating worsen.
- Poor sleep on hot nights is itself a seizure trigger, so keep the bedroom cool.
- Report increased seizure frequency, confusion or marked weakness.
Sources
Benzodiazepines and Z-drugs
Moderate riskmoderate evidenceWhy heat is harder
- Sedation dulls awareness, so early signs of overheating and thirst are missed.
- Blood pressure drops more readily, so falls are more likely in the heat.
What helps
- Sedation dulls awareness, so early signs of overheating and thirst are missed.
- Keep drinking water within reach and ask someone to check on you during a heat alert.
- Falls are more likely when sedation combines with low blood pressure from heat.
- Avoid alcohol, which adds to both the sedation and the dehydration.
Sources
Opioid painkillers
Moderate riskmoderate evidenceWhy heat is harder
- These act on the brain's temperature control and blunt awareness of overheating.
- Some have anticholinergic effects so sweating is reduced, and vomiting or constipation adds to fluid loss.
- Patches absorb faster from hot skin, so the dose delivered can rise unexpectedly.
What helps
- Patches absorb faster when the skin is hot, so keep patches out of the sun and away from hot baths and heat pads.
- Drowsiness makes it harder to recognise heat exhaustion and to act on it.
- Tramadol also acts on serotonin, so combined with heat there is a small risk of a dangerous temperature rise.
- Get help for confusion, very slow breathing or extreme drowsiness.
Sources
Gabapentin and pregabalin
Lower risklimited evidenceWhy heat is harder
- Drowsiness and dizziness blunt awareness of overheating and make falls more likely.
- Both are cleared by the kidneys, so dehydration raises levels, adds to sedation and lowers blood pressure.
What helps
- Drowsiness and dizziness are the main concerns, making falls more likely in the heat.
- Both are cleared by the kidneys, so dehydration can raise levels and increase drowsiness.
- Stand up slowly and keep to cool spaces when temperatures are high.
- Report unusual sleepiness or unsteadiness.
Sources
Triptans
Lower risklimited evidenceWhy heat is harder
- These narrow blood vessels, which slightly reduces the skin's ability to shed heat.
- They act on serotonin, which the brain uses to help set body temperature.
What helps
- Combined with other serotonin medicines there is a small risk of a dangerous temperature rise.
- Dehydration is itself a migraine trigger, so drinking regularly helps on both counts.
- Rest somewhere cool and dark when treating a migraine in hot weather.
Sources
NSAIDs
Higher riskstrong evidenceWhy heat is harder
- When you are dehydrated these cut blood flow into the kidney, so kidney injury is much more likely.
- They cause salt and water retention, which strains the heart and circulation.
What helps
- When you are dehydrated these markedly raise the risk of kidney injury, most of all alongside an ACE inhibitor or ARB and a diuretic.
- Avoid taking them for heat-related headache or unusual pains during a heatwave.
- Ask a pharmacist about a safer alternative for pain relief in hot weather.
- Report much reduced urine output, ankle swelling or unusual tiredness.
Sources
Aspirin
Moderate riskmoderate evidenceWhy heat is harder
- Dehydration raises blood levels, and at painkiller doses aspirin stresses the kidneys.
- High doses interfere with the body's normal adjustment to a raised temperature.
What helps
- At painkiller doses aspirin can interfere with the body's adjustment to heat and stress the kidneys.
- French guidance advises against using aspirin for fever or unusual pain during a heatwave.
- Low dose aspirin taken to protect the heart should be continued as prescribed.
- Ask a pharmacist before using high dose aspirin in hot weather.
Sources
HRT and hormonal contraceptives
Lower risklimited evidenceWhy heat is harder
- Patches and gels absorb faster from hot skin, and sweating can loosen a patch.
- Oestrogen slightly raises the cardiovascular clot risk, which dehydration and sitting still add to.
What helps
- HRT generally reduces hot flushes, so it often makes heat easier rather than harder to cope with.
- Keep patches out of direct sun, check they are still stuck down, and store them below 25C.
- Dehydration and long periods of sitting still add to clot risk, so drink water and move about.
Sources
GnRH agonists, tamoxifen and aromatase inhibitors
Moderate riskmoderate evidenceWhy heat is harder
- These act on the brain's temperature control, commonly causing hot flushes.
- Night sweats and flushing increase fluid loss and badly disturb sleep.
What helps
- Tell the oncology team if flushes become unmanageable, as treatments can help.
- Wear loose layers of natural fibres and keep a fan or cool water spray to hand.
- Extra sweating means extra fluid loss, so drink more than usual.
- Sleep is often badly affected, so cool the bedroom in the evening before bed.
Sources
Corticosteroids
Moderate risklimited evidenceWhy heat is harder
- These alter salt and water balance, so fluid losses are harder to replace.
- Blood pressure and blood glucose both shift, which worsens dehydration.
- Sweating can increase, adding to fluid loss.
What helps
- People on long term steroids may need extra doses when acutely unwell, so follow the sick day rules given.
- Carry a steroid emergency card and know when to seek urgent help.
- Monitor blood glucose if you have diabetes, as steroids raise it further.
- Keep fluids up, as steroids alter salt and water balance.
Sources
Immunosuppressants
Higher riskmoderate evidenceWhy heat is harder
- Dehydration raises blood levels of ciclosporin and tacrolimus, which can harm the kidneys.
- Methotrexate and others cause photosensitivity, so skin burns after little sun.
What helps
- Contact the specialist team if you cannot keep fluids down.
- Expect more frequent blood level and kidney function checks in hot spells.
- Methotrexate causes marked sun sensitivity, so use high factor sunscreen and cover up.
- Many of these medicines raise long term skin cancer risk, so sun protection matters year round.
Sources
Chemotherapy agents
Higher riskmoderate evidenceWhy heat is harder
- Vomiting and diarrhoea cause fluid loss that heat makes worse.
- Dehydration concentrates these medicines, and several are toxic to the kidneys.
- Many cause photosensitivity, and fatigue, low blood counts and cardiovascular side effects leave little reserve for heat.
What helps
- Contact the oncology team promptly if you become dehydrated or develop a fever.
- Several agents are toxic to the kidneys, so staying hydrated protects kidney function.
- Many cause strong sun sensitivity, so use high factor sunscreen, a hat and cover up.
- Fatigue and low blood counts reduce the ability to cope with heat, so rest in cool rooms.
Sources
Photosensitising antibiotics
Moderate riskstrong evidenceWhy heat is harder
- These cause photosensitivity, so skin burns after only brief sun exposure.
- Diarrhoea and vomiting add to fluid loss and dehydration.
What helps
- Doxycycline in particular causes severe sunburn-like reactions after only brief sun exposure.
- Use broad spectrum sunscreen of at least SPF 30, a hat and long sleeves, and avoid sunbeds entirely.
- Report any painful or blistering rash on sun exposed skin.
- Diarrhoea and vomiting from antibiotics also cause dehydration, so drink plenty.
Sources
Isotretinoin and other retinoids
Moderate riskstrong evidenceWhy heat is harder
- Retinoids thin the outer skin layer and cause photosensitivity, so sunburn comes easily.
- Dry skin, eyes and lips are worse in the heat, and dehydration adds to the dryness.
What helps
- Retinoids thin the outer skin layer and make sunburn much more likely, so use high factor sunscreen daily.
- Dry skin, eyes and lips are worse in the heat, so use plenty of moisturiser and lip balm.
- Avoid sunbeds and waxing while on treatment.
- Drink plenty of fluids, as dehydration worsens the dryness.
Sources
Amiodarone
Higher riskstrong evidenceWhy heat is harder
- Amiodarone causes marked photosensitivity that persists for months after stopping.
- Dehydration changes blood levels of a medicine with a narrow safety margin.
- It can alter thyroid function in either direction, which changes heat tolerance.
What helps
- Use high factor sunscreen and cover exposed skin, as the sun sensitivity persists for months after stopping.
- It can leave a lasting blue-grey skin discolouration, and can alter thyroid function either way.
- Dehydration can change blood levels, so keep fluids up and attend monitoring blood tests.
- Report palpitations, marked breathlessness or a new rash.
Sources
Digoxin
Higher riskstrong evidenceWhy heat is harder
- Digoxin has a narrow safety margin, and dehydration or low potassium tips it into toxicity.
- It slows the heart, so the circulation cannot speed up to shed heat.
What helps
- Digoxin has a narrow safety margin, and dehydration or low potassium raises the risk of toxicity.
- Keep fluid intake steady, particularly if you also take a diuretic.
- Report nausea, vomiting, visual disturbance, confusion or a very slow pulse urgently.
- A digoxin level and potassium check may be advised during a prolonged hot spell.
Sources
Anti-Parkinson's medicines
Higher riskstrong evidenceWhy heat is harder
- Anticholinergics such as trihexyphenidyl and amantadine cause much reduced sweating.
- Parkinson's medicines act on the brain pathways that set body temperature, and blood pressure often drops in heat.
- Missed or suddenly stopped doses can cause a dangerous rise in temperature.
What helps
- Never miss or suddenly stop doses, as abrupt withdrawal can cause a dangerous rise in temperature.
- Amantadine and anticholinergics such as trihexyphenidyl reduce sweating considerably.
- Blood pressure often drops in the heat with these medicines, so move slowly and sit down if dizzy.
- Get urgent help for a high temperature with marked muscle stiffness or confusion.
Sources
Decongestants
Moderate risklimited evidenceWhy heat is harder
- These narrow blood vessels, including the skin vessels that carry heat away.
- They have mild anticholinergic effects, so sweating is reduced.
- Pulse rate and heat production both rise.
What helps
- Avoid oral decongestants on very hot days if you can manage without them.
- They narrow skin blood vessels and raise blood pressure and heart rate, adding to the strain of heat.
- Nasal sprays act mainly on the nose and are a lower risk option, so ask a pharmacist.
Sources
Caffeine
Lower riskmoderate evidenceWhy heat is harder
- Very high doses increase urine output, though ordinary amounts do not dehydrate regular users.
- Caffeine raises pulse rate and the heat the body produces.
What helps
- Moderate caffeine in ordinary drinks does not cause meaningful dehydration for regular users.
- Very high doses, such as several energy drinks, do increase urine output and heart rate.
- Avoid using caffeine to push through fatigue during exertion in high temperatures.
Sources
Alcohol
Higher riskstrong evidenceWhy heat is harder
- Alcohol increases urine output, so fluid is lost on top of sweating.
- It blunts judgement about how hot you are and whether to do anything about it.
- Widened skin vessels feel cooling but speed heat gain when the air is hotter than the skin, and blood pressure falls.
What helps
- Drink water alongside alcohol and keep quantities lower than usual in hot weather.
- Widened skin vessels feel cooling but combined with heat can drop blood pressure and cause fainting.
- Falling asleep outdoors or in the sun after drinking is a common route to serious heat illness.
- Alcohol worsens dehydration risk with diuretics, lithium and many other medicines.
Sources
Cannabis
Moderate risklimited evidenceWhy heat is harder
- Cannabis affects the brain pathways that set body temperature.
- It raises heart rate and lowers blood pressure, so fainting is more likely in heat.
- It blunts awareness of how hot you are and how much you have drunk.
What helps
- Cannabis raises heart rate and can lower blood pressure, so fainting is more likely in heat.
- It also blunts awareness of how hot you are and how much you have drunk.
- Stay somewhere cool and shaded.
- Get help for a rapid heartbeat with confusion or a high temperature.
Sources
MDMA and recreational stimulants
Higher riskstrong evidenceWhy heat is harder
- These stimulants raise body temperature directly and substantially, and narrow skin blood vessels so heat cannot escape.
- They disrupt the brain's temperature control, which is the main cause of serious harm.
- Thirst and fatigue cues are blunted, so people push on far too long while sweating heavily.
What helps
- Overheating is the main cause of serious harm and death with MDMA, not the dose alone, and hot crowded venues multiply the effect.
- Take regular breaks in cool air and rest rather than dancing continuously.
- Sip water steadily rather than drinking very large volumes quickly, which can cause dangerously low sodium.
- A very high temperature with confusion, rigid muscles or fitting is a medical emergency, so call 999.
Sources
Nicotine
Lower risklimited evidenceWhy heat is harder
- Nicotine narrows small blood vessels, so slightly less heat escapes through the skin.
- It raises pulse rate and heat production a little.
- Patches absorb faster from hot sweaty skin and may lift off.
What helps
- Nicotine patches can absorb faster from hot sweaty skin, so keep them out of direct sun.
- Patches may also lift off when you sweat, so check they are still stuck down.
- Remove a patch and seek advice if you feel nauseous, dizzy or have palpitations.
Sources
Antidepressant-driven excessive sweating
Moderate riskmoderate evidenceWhy heat is harder
- Sweating far more than usual affects a substantial minority of people on these medicines.
- The extra sweat means extra fluid and salt loss, and patches or sensors may not stick.
What helps
- If sweating is intolerable, ask about a dose review or a different medicine rather than stopping.
- It increases fluid and salt loss, so drink more than usual on hot days.
- Wear loose breathable clothing and carry a spare top if sweating is heavy.
- Sweating can also loosen any skin patches and affect glucose sensor adhesion.
Sources
Beta-agonist inhalers
Lower risklimited evidenceWhy heat is harder
- These raise heart rate and cause tremor, which feels worse in the heat.
- High doses lower potassium, which matters more when you are dehydrated.
What helps
- Heat, humidity, ozone and pollen all trigger asthma, so carry a reliever inhaler at all times.
- Never leave an inhaler in a hot car, as pressurised canisters can be damaged or burst.
- High doses lower potassium levels, which matters more if you are also dehydrated.
- Seek urgent help if a reliever inhaler is not easing breathlessness.
Sources
Nitrates and GTN spray
Moderate riskmoderate evidenceWhy heat is harder
- Nitrates widen blood vessels to relieve angina. Heat already widens them, so the two together can drop your blood pressure further than usual.
- A GTN spray works fast, so the fall in blood pressure comes quickly. Dizziness and fainting are more likely on a hot day.
What helps
- Sit down before using your GTN spray in hot weather, rather than standing.
- Keep taking your nitrates as prescribed. Tell your GP if you are getting dizzy, rather than stopping them yourself.
- Get up slowly from sitting or lying, especially after a hot bath or shower.
- Keep the spray out of the car and out of direct sun. Most medicines need storing below 25C, so check the label.
Sources