The UK Health Security Agency (UKHSA) has issued a warning that the ongoing heatwave could silently ruin medicines, leaving life-saving drugs ineffective. The Medicines and Healthcare products Regulatory Agency (MHRA) says everything from asthma inhalers and insulin to hormone patches and EpiPens may lose their potency when exposed to high temperatures.
Most medicines are designed to be stored below 25°C. However, watchdogs warn that cars, buses, and trains can effectively turn into ovens within minutes during a heatwave, while even a warm handbag or trouser pocket left in the sun could compromise vital treatments.
Insulin and Antibiotics at Risk
Insulin and certain liquid antibiotics, which must be refrigerated, are particularly at risk. Officials warn they may simply stop working if left out in the heat for too long. Alison Cave, the MHRA's chief safety officer, said many people are so focused on enjoying the sunshine that they overlook the risk. She admitted medicines left baking in the heat "might not work properly when you need them" and cautioned that some drugs can also leave patients more likely to burn, feel dizzy, or become dangerously dehydrated.
She urged the public to keep medicines somewhere cool and shaded, stay alert to signs of heat-related illness, and take extra care in the sun if their medication increases the risk of burning.
Hidden Damage and Device Risks
Officials warn that heat-damaged medicine may not always look different, though patients should check for changes in colour, smell, or texture. Blood glucose monitors used by diabetics can also be affected by heat and humidity, potentially delivering unreliable readings.
Dehydration Danger for Those on Blood Pressure Pills
Millions taking diuretics, or "water tablets", such as furosemide, are already losing extra fluid through sweat and urination, raising the risk of dehydration and dangerous mineral imbalances in extreme heat. Common blood pressure drugs, including ACE inhibitors, beta-blockers, and calcium channel blockers, can also make it harder for the body to regulate its temperature. ACE inhibitors may blunt the natural thirst response, leaving patients unaware they need to drink more.
Those on insulin or metformin for diabetes face a heightened dehydration risk, while patients taking antipsychotic drugs such as olanzapine and quetiapine, or stimulant medication for attention disorders, may find their bodies struggle to cool down.
Warning Signs of Dehydration
Dizziness, headaches, tiredness, confusion, and dark urine are among the warning signs of dehydration, problems considered especially dangerous for the elderly, young children, and those with heart or kidney conditions.
Sunburn Risk from Common Tablets
Patients have also been warned that everyday medicines could leave their skin more vulnerable to sunburn, even on a mildly sunny day. Certain antibiotics, diuretics, antidepressants, and acne or eczema treatments are among those flagged, alongside painkillers such as ibuprofen and naproxen. Officials stress this risk is more commonly linked to frequent or long-term use.
Particular concern surrounds methotrexate, a drug used to treat rheumatoid arthritis, psoriasis, Crohn's disease, and cancer. It can trigger extreme sun sensitivity even at low doses, potentially causing painful blistering rashes resembling severe sunburn. In rare cases, this has led to serious infections. Even a short dog walk, gardening, or a sunny commute could trigger a reaction in vulnerable patients.
Anyone concerned about their medication should check the patient information leaflet or speak to their pharmacist, and report any suspected problems through the MHRA's Yellow Card scheme.
