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Heatwaves and medicines: assessing and managing patient risk

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Hot weather can affect the body’s responses to some medicines and medical devices. Identify people at risk and manage the potential impact.

Contents

  1. Safety alert
  2. Check correct storage
    1. Use our advice
  4. Step wise approach
    1. Identify people at increased risk
    2. Assess medicine related risk
    3. Give advice when using medicines
  6. Specific medicine considerations
    1. ACE inhibitors and ARBs
    2. Antibiotics and antifungals
    3. Anticholinergics
    4. Antidepressants
    5. Antiplatelets
    6. Antipsychotics
    7. Antiseizure
    8. Beta blockers
    9. Calcium channel blockers
    10. Diuretics
    11. Insulin
    12. Levothyroxine
    13. Metformin
    14. Methotrexate
    15. Narrow therapeutic index
    16. NSAIDs
  8. Specific medical device considerations
    1. Adrenaline auto injectors (AAIs)
    2. Blood glucose monitors and test strips
    3. Continuous glucose monitors (CGMs)
    4. Inhalers

Safety alert

Hot weather changes how the body responds to some medicines, which could impact people managing long-term conditions.

The MHRA advise people to:

  • store medicines somewhere cool, dry and out of direct sunlight
  • stay hydrated
  • use sun protection if taking medicines which increase sensitivity to sunlight

For many medicine classes, evidence is limited, inconsistent, or based on indirect mechanisms such as dehydration rather than direct effects on temperature regulation.

In June 2026 the MHRA issued Don’t let the heatwave affect your medicines to highlight medicines that may increase heat-related risks.

Check correct storage

Correct storage maintains medicine quality, effectiveness and safety.

Always refer to the manufacturer’s guidance on the packaging to check product storage conditions.

Use our advice

Follow our advice to assess the significance of the temperature excursion on medicines stability as well as guidance on controlling ambient storage temperatures.

Managing temperature excursions

Where temperature excursions occur, follow a defined process to manage and mitigate the effects on medicines

Step wise approach

Assess the risk of hot weather on a person already taking medicines.

Organisations are encouraged to review these steps and consider incorporating into local processes.

  1. Assess the person before focusing on the medicine.

    Consider whether the person has factors that make them more vulnerable to heat-related illness.

    Risk factors include:

    • cardiovascular disease
    • diabetes
    • fluid restricted
    • infants and children
    • kidney disease
    • limited access to cooling or adequate hydration
    • older people
    • polypharmacy
    • pregnant or breastfeeding people
    • severe mental illness or dementia

  2. Assess whether medicines affect the person’s ability to cope with hot weather.

    Medicines may increase risk by:

    • accumulating during dehydration
    • altering thermoregulation
    • causing electrolyte disturbances
    • causing sedation or impaired awareness
    • increasing dehydration
    • altering sweating
    • increasing photosensitivity
    • increasing the risk of hypotension and falls

    Check the Summary of Product Characteristics (SmPC) via the Electronic Medicines Compendium (eMC) to find relevant cautions, contraindications or side effects.

    Risk depends on the individual, their comorbidities, living environment and medicine regimen.

  3. Do not recommend stopping medicines routinely during hot weather.

    Most people at increased risk will require advice and reassurance. They are likely to require closer self-monitoring rather than medicine changes.

    Advise anyone taking medicines to:

    • continue prescribed medicines
    • stay hydrated unless fluid restriction has been advised
    • keep cool and avoid excessive heat exposure
    • follow medicine and device storage requirements
    • use sun protection when using photosensitising medicines

Specific medicine considerations

Consider medicine-related risk alongside age, comorbidities, frailty and environmental factors.

The following list of medicines gives potential mechanisms for how medicines can interfere with the body’s response to hot weather. The list is not exhaustive.

Advise people to maintain fluid intake where appropriate and seek review if they develop dizziness, postural symptoms, reduced urine output or signs of dehydration.

These drug classes may lower blood pressure and reduce thirst sensation, increasing the risk of dehydration, fainting and falls.

Antibiotics such as ciprofloxacin, doxycycline, metronidazole, and tetracycline, and antifungals such as itraconazole, griseofulvin, and voriconazole.

Advise sun avoidance where possible, the use of protective clothing and hats, and broad-spectrum sunscreen (SPF 30 or higher). Review if severe skin reactions occur.

These medicines may increase sensitivity to sunlight and cause a sunburn-like skin reaction (photosensitivity).

Advise people to stay cool and hydrated.

This drug class may reduce sweating and impair the body’s ability to regulate temperature, increasing the risk of heat exhaustion and heat stroke. They can also worsen confusion and cognitive impairment.

The degree of effect varies between anticholinergics and depends on their anticholinergic receptor affinity, ability to cross into the central nervous system, dose, and route of administration.

Advise people to keep cool, maintain adequate hydration, and seek medical advice if symptoms of heat-related illness develop.

Selective serotonin reuptake inhibitors (SSRIs), serotonin–noradrenaline reuptake inhibitors (SNRIs), and some tricyclic antidepressants (TCAs) can increase sweating, which may contribute to dehydration and electrolyte disturbances. In addition, TCAs and monoamine oxidase inhibitors (MAOIs) may have anticholinergic effects that impair thermoregulation and reduce the ability to adapt to heat, increasing the risk of heat-related illness.

Advise people to stay hydrated, avoid prolonged exposure to excessive heat and seek review if symptoms of heat-related illness occur.

This drug class may reduce superficial vasodilation and affect heat dissipation.

Advise carers to seek urgent medical attention if symptoms of heat stroke develop.

This drug class may interfere with thermoregulation and reduce the body’s ability to respond to heat.

Such as topiramate, oxcarbamazepine and carbamazepine.

Advise people to maintain hydration and avoid excessive heat exposure. Review if seizure frequency increases or symptoms of dehydration develop.

Some antiseizure medicines may reduce sweating, increase urination, and cause dizziness or weakness.

Some people with epilepsy may have temperature sensitive seizures.

Advise people to rise slowly from sitting or lying positions and seek review if symptoms worsen.

This drug class may reduce sweating and vasodilation, lower blood pressure and increase the risk of fainting and falls.

Advise people to maintain hydration and seek review if they develop dizziness, postural symptoms or signs of dehydration.

This drug class may lower blood pressure and increase the risk of fainting and falls.

Consider monitoring renal function and electrolytes where there is a suspicion of Acute Kidney Injury (AKI).

Diuretics may cause dehydration, volume depletion and electrolyte imbalance. Reduced thirst sensation may further increase dehydration risk.

If blood sugar levels are consistently higher than expected, then the insulin may have been damaged by the heat and should be replaced.

Advise people to store insulin in the fridge, cool bag or similar. Pump reservoirs may need changing more frequently.

Heat can also increase the absorption of insulin which may lead to hypoglycaemia.

People with diabetes can get useful advice from Diabetes.UK which suggests reducing risk by:

  • checking blood sugar levels more regularly
  • keeping blood glucose meters and test strips close to room temperature and out of direct sunlight
  • following manufacturers instructions when using continuous glucose monitors

Advise people to maintain hydration. Consider reviewing thyroid control if symptoms suggest over-replacement due to dehydration.

Levothyroxine may contribute to excessive sweating and heat intolerance.

Advise people to maintain hydration and seek medical advice if they develop vomiting, diarrhoea or symptoms of dehydration.

Metformin may increase the risk of dehydration, and early symptoms may be difficult to recognise.

Advise sun avoidance where possible, the use of protective clothing and hats, and broad-spectrum sunscreen (SPF 30 or higher). Review if severe skin reactions occur.

Methotrexate may cause severe sun sensitivity (photosensitivity) resulting in painful rashes, blistering or swelling.

Consider whether clinical review or therapeutic drug monitoring is required if people present with signs of toxicity. Advise people to maintain hydration and seek medical advice if they develop toxicity signs.

Dehydration and electrolyte imbalance may increase serum concentrations and toxicity risk.

Advise people to maintain hydration and seek medical advice if they develop signs of Acute Kidney Injury (AKI). See the Clinical Knowledge Summary on when to suspect AKI for examples.

NSAIDs may increase the risk of AKI during periods of dehydration.

Specific medical device considerations

Heat exposure may damage medical devices. Always follow manufacturer’s storage advice. This list is not exhaustive.

Advise people to regularly inspect their AAIs for damage or discolouration. In anaphylaxis, do not delay the use of an available AAI because of concerns about previous heat exposure or expiry if no suitable alternative is immediately available.

Heat and direct sunlight may affect adrenaline stability and AAI performance.

Consider a control check if results do not match symptoms.

Heat exposure may affect the accuracy of blood glucose monitors and test strips.

Advise people to avoid getting any sunscreen on the site of insertion when changing the CGM. The site should always be thoroughly cleaned before applying a new sensor.

Remind people that interstitial fluid glucose readings are a few minutes behind blood glucose levels, and finger-prick checks may be required if symptoms do not match CGM reading.

Increased sweating may affect sensor adhesion.

Advise people not to leave inhalers in parked vehicles or other hot environments.

Excessive heat may damage inhalers and, in some cases, cause pressurised inhalers to burst.