With the UK bracing for another heatwave this week, the Medicines and Healthcare products Regulatory Agency (MHRA) is reminding people that high temperatures can affect medicines and how well they work. Hot weather changes how your body responds to medications, which could impact people managing long-term conditions – but a few simple steps can help avoid problems.
Alison Cave, Chief Safety Officer at the MHRA, said:
“Let’s face it – when there’s a heatwave, most of us are focused on getting outside and enjoying it while it lasts. But it’s easy to forget that medicines left in the heat – in cars, bags, or on sunny windowsills – might not work properly when you need them.
“Some medicines can also make you more likely to burn in the sun, feel dizzy, or get dehydrated, especially if you’re taking diuetics or have a condition like asthma, heart disease, or diabetes.
“To stay safe in the heat:
- Store medicines somewhere cool, dry and out of direct sunlight – especially if you’re out and about
- 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
“And remember, for all medicines it’s important to read the leaflet and speak to a healthcare professional if you have any questions.”
1. Keep medicines out of hot cars and direct sunlight
Most medicines need to be stored at temperatures below 25°C. But cars, trains and buses can heat up quickly during the summer, and rooms with direct sunlight can get much hotter than you’d think. Even bags and pockets can get warm enough to cause problems, especially if you’re out all day or at a festival, sports event, or the beach.
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.
Medicines that need to be kept in the fridge – such as insulin or certain liquid antibiotics – are especially vulnerable to heat. If they’re left out too long or stored incorrectly, they may not work properly when you take them.
If your medicine has changed colour, developed an unusual smell, changed texture, or looks different in any way, this could signal heat damage. Ask your pharmacist for advice.
If you’re at home, check the patient information leaflet for storage instructions and keep medicines in the coolest part of the house, away from direct sunlight. If you’re travelling, avoid leaving them in parked vehicles or warm bags. A cool bag can help if you’re going to be out all day.
Heat damage can also affect how much medicine your body absorbs, potentially making your usual dose too strong or too weak. 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.
Don’t forget medical devices:
It’s not just medicines – some medical devices can also be affected by heat or humidity. For example, blood glucose monitors may give people managing diabetes less reliable readings if used or stored in hot conditions.
Keep devices and any test strips somewhere cool and dry, and check the instructions that came with them for any temperature guidance. If your device isn’t behaving as expected, try running a control check or speak to your pharmacist for advice.
For more information about medicine safety, visit the MHRA website or report any concerns through the MHRA Yellow Card scheme.
2. Drink more water and stay cool if you’re taking water tablets or blood pressure medication
During hot weather, your body loses more water through sweating – but some medicines can make this worse, or make it harder for your body to cope with heat.
People taking diuretics, or ‘water tablets’, such as furosemide, are already losing more fluid through urination. This can cause dehydration and mineral imbalances. Blood pressure medicines such as ACE inhibitors, beta-blockers, or calcium channel blockers can also make it harder for your body to regulate temperature, especially during sudden hot spells. ACE inhibitors can even suppress your natural thirst response.
Some diabetes medicines, including insulin and metformin, also increase the risk of dehydration or make early symptoms harder to spot. Antipsychotic medications, such as olanzapine or quetiapine, or stimulant medications for attention disorders can increase your body temperature, making you overheat more easily.
Dehydration can cause dizziness, headaches, tiredness, confusion, and dark urine. This is particularly dangerous for older adults, young children, and people with heart or kidney conditions. If you start to feel unwell in the heat, move somewhere cooler, sip water slowly, and seek medical advice if you don’t feel better quickly.
3. Check if your medicine increases your risk of sunburn
Some common medicines can make your skin much more likely to burn, even in mild sunshine. This includes some antibiotics, diuretics, antidepressants, or skin treatments for acne or eczema. Painkillers such as ibuprofen or naproxen are also known to sometimes cause mild sun sensitivity, but this is less common and usually only a concern with frequent or long-term use.
One medicine that requires particular care is methotrexate, used to treat conditions like rheumatoid arthritis, psoriasis, Crohn’s disease and cancer. It can cause extreme skin sensitivity to the sun – even at low doses. This can lead to painful rashes, blistering, or swelling that looks like severe sunburn. In rare cases, these reactions have led to serious infections.
The risk goes up during sunny weather, especially if you’re outside more – whether you’re walking the dog, gardening, or heading off on holiday. Even a short walk at lunchtime or a train ride in the sun can be enough to trigger a reaction for some people.
If you take any medicines, always read the patient information leaflet or check with your pharmacist to see if sun sensitivity is a side effect.
Notes to editors
- Report any concerns through the MHRA Yellow Card scheme, which can be accessed at https://yellowcard.mhra.gov.uk/
- The Medicines and Healthcare products Regulatory Agency (MHRA) is responsible for regulating all medicines and medical devices in the UK by ensuring they work and are acceptably safe. All our work is underpinned by robust and fact-based judgements to ensure that the benefits justify any risks.
- The MHRA is an executive agency of the Department of Health and Social Care.
- For media enquiries, please contact the newscentre@mhra.gov.uk, or call on 020 3080 7651.