The Medicines and Healthcare products Regulatory Agency (MHRA) has reissued its "essential advice" for people taking inhalers, insulin, EpiPens and other common medicines, warning that heat can break down active ingredients and stop them working. The guidance was reissued on 26 June during a heatwave that peaked at a record 37.7C, and is again relevant as temperatures are expected to exceed 30C in parts of the UK on Monday, with 33C forecast in London and 29C in Cardiff.
Most medicines must be kept below 25C, which can prove difficult in cars, on public transport, and even in bags or pockets as well as in homes and workplaces.
How heat affects medication
The MHRA warns: "Heat can break down the active ingredients in medicines, meaning they might not work when you need them. That includes tablets, inhalers, hormone patches, insulin, and EpiPens. Some packaging will give clear storage instructions, but not all medicines come with a visible warning."
It says that "medicines that need to be kept in the fridge – such as insulin or certain liquid antibiotics – are especially vulnerable to heat" and that if they are left out for too long or stored improperly "they may not work properly when you take them".
How to tell if heat has damaged your medicine
According to the MHRA, a change in colour, an unusual odour, a different texture, or any other visible change can signal heat damage. It advises speaking to a pharmacist.
The agency also cautions that heat can affect how much medicine your body absorbs, making a normal dose either too potent or too weak. It says: "If you notice any changes in how you feel or how well your medicines seem to be working during hot weather, speak to your pharmacist or doctor."
Medication and heat: extra risks
Some medicines make the body more vulnerable in hot weather. Insulin and metformin increase the risk of dehydration and can make early warning signs harder to spot, while ACE inhibitors and beta-blockers can hinder the body's ability to regulate temperature. ACE inhibitors can even suppress the natural thirst response.
Hot weather and asthma
Asthma UK warns that hot weather is a common trigger, with one in three people saying it made their symptoms worse. Hot, humid air can cause breathlessness and wheezing, and hot sunny weather increases ozone pollution. The charity advises people with asthma to always carry their inhaler and keep it out of direct sunlight and away from heat.
The NHS also notes that high temperatures often bring elevated pollen counts and poorer air quality, both of which can aggravate asthma symptoms.
Hot weather and diabetes
Diabetes UK warns that "sitting in the sun for long periods may affect your diabetes because you're not being very active, making blood sugar levels higher than usual". It adds that "if you take insulin to treat your diabetes it may be absorbed more quickly from the injection site in warm weather, and this increases the risk of hypos". It advises people to check their blood sugar levels more often.
How to keep medicines safe in hot weather
- Store medicines somewhere cool, dry and out of direct sunlight
- Avoid leaving them in cars or in bags left in sunlight
- Know the signs of heat-related illness
- Stay hydrated and listen to your body
- Take extra care in the sun if your medicine makes your skin more likely to burn
- Read the leaflet and speak to a healthcare professional if you have any questions
