Warm summer weather can change more than your daily routine. Some prescription and over-the-counter medicines can affect how the body responds to heat, how easily it becomes dehydrated, or how sensitive the skin is to sunlight.

The Medicines and Healthcare products Regulatory Agency is reminding people to check their medication information and take extra care during periods of high temperatures.

Some Medicines Increase Dehydration Risk

Hot weather already causes the body to lose more fluid through sweating. Certain medicines can make that process more significant or interfere with normal temperature regulation.

Diuretics, which are commonly prescribed for high blood pressure and some heart conditions, increase fluid loss through urination. During hot weather, this may increase the likelihood of dehydration if fluid intake does not keep pace.

Some antidepressants and antipsychotic medicines can interfere with the body's ability to regulate temperature, while beta blockers may reduce some of the normal responses that help the body cope with heat stress.

Possible signs that the body is struggling include dizziness, headache and unusual tirednessl. These symptoms should not automatically be blamed on the weather, particularly if they are persistent or becoming worse.

Medicines Can Make Skin More Sun-Sensitive

Another issue is photosensitivity — an unusually strong skin reaction to sunlight.

Someone taking a photosensitising medicine may burn more quickly or develop redness, soreness or blistering after an amount of sun exposure that would not normally cause such a reaction.

Certain antibiotics, including doxycycline and tetracycline, are among the medicines known to increase photosensitivity. Doxycycline is specifically associated with increased sensitivity to sunlight in NHS prescribing information.

Some acne treatments can have the same effect, including isotretinoin and topical retinoids. The antifungal medicine voriconazole and some disease-modifying antirheumatic medicines, including methotrexate and azathioprine, can also increase sensitivity to sunlight.

Even medicines that may seem unrelated to skin can occasionally be involved. The MHRA notes that proton pump inhibitors used for heartburn and acid reflux have been linked to a very low risk of a lupus-like skin reaction triggered by sunlight. Common painkillers including ibuprofen and naproxen may also increase sun sensitivity in some people.

Protect Yourself From Strong Sun

People taking medicines associated with photosensitivity do not necessarily need to avoid going outdoors completely.

Simple precautions can reduce exposure considerably.

Try to limit time in strong midday sun, seek shade where practical and wear clothing that covers exposed skin. Sunscreen should be used according to the instructions on the product and reapplied when necessary.

Pay particular attention if your skin suddenly reacts more strongly than usual. Redness, soreness or blistering after relatively limited sun exposure could indicate a medicine-related reaction.

Stay Hydrated — But Follow Medical Advice

Regular fluid intake is important during hot weather, especially when taking medicines that increase fluid loss.

However, people who have been given specific fluid restrictions because of heart, kidney or another medical condition should continue following their individual medical advice rather than suddenly increasing intake on their own.

Moving into cooler rooms, avoiding strenuous activity during the hottest part of the day and limiting unnecessary exposure to high temperatures can also reduce heat stress.

Do Not Stop Medication Yourself

One of the most important messages is that hot weather is not a reason to stop prescribed treatment.

Suddenly discontinuing some medicines can create much greater health risks than continuing them during a heatwave.

Dr Alison Cave, Chief Safety Officer at the MHRA, advises people to understand how their medicines may affect hydration, temperature regulation or sun sensitivity and to use simple protective measures rather than altering treatment without professional advice.

If you are uncertain whether a particular medicine has heat- or sunlight-related precautions, check the patient information leaflet supplied with it or ask a pharmacist or GP. The NHS also provides individual information for many commonly used medicines.

Keep Medicines Out of Excessive Heat

Heat can affect the medicine itself as well as the person taking it.

The MHRA advises against leaving medicines in hot cars or direct sunlight. Many products have specific storage temperature requirements, and vehicles or bags left in strong sunshine can become much hotter than expected.

Follow the storage instructions on the packaging or leaflet, particularly for temperature-sensitive medicines.

Know When to Ask for Help

Seek medical advice if you develop an unusual skin reaction, significant dizziness, confusion, persistent weakness or other symptoms that concern you during hot weather.

If you suspect that a medicine has caused a side effect, speak with a doctor, pharmacist or nurse. Suspected adverse reactions can also be reported through the MHRA Yellow Card system.