Photosensitivity: Medicines That Can Cause Reactions to Sunlight
Summer brings long days, high temperatures and a natural desire to spend time outdoors. Between trips to the beach, walks and late-afternoon visits to outdoor cafés, it is easy to forget that excessive heat can have unexpected effects, particularly on people taking certain medicines.
In fact, some commonly used medicines can affect how the body responds to heat or sun exposure, increasing the risk of dehydration, heatstroke or severe skin reactions. In this article, we explain why this happens, identify the medicines most commonly involved and provide practical advice on how to protect yourself.
The body adapts to heat—but not always.
Before discussing specific medicines, it is important to understand how the body naturally responds to heat. When temperatures rise, the body produces sweat, blood flow to the skin increases, and we feel the need to seek shade and drink water. These are protective mechanisms that help keep the body’s internal temperature stable.
However, certain substances can interfere with these processes, making the body more vulnerable. The link between heat and medicines may not be immediately apparent, but its effects can be dangerous, especially for at-risk groups such as older adults, children and people with chronic conditions.
Which medicines interfere most with the body’s ability to regulate heat?
In practice, several groups of medicines can impair the body’s ability to cope with high temperatures. Some increase fluid loss, while others reduce sweating or alter the sensation of thirst.
The most relevant include:
- Diuretics: Increase the elimination of water and electrolytes, which can quickly lead to dehydration and low blood pressure.
- Psychotropic medicines: Antipsychotics and tricyclic antidepressants can interfere with the central mechanisms that regulate body temperature.
- Medicines for cardiovascular conditions: Beta-blockers and ACE inhibitors can interfere with the heart’s and blood vessels’ ability to adapt to the strain caused by high temperatures.
- Antihistamines: Although used to treat allergies, they can reduce sweat production, which is essential for cooling the body.
- Anticholinergic medicines: Used to treat conditions such as overactive bladder, nausea or neurological disorders, these medicines can also impair sweating and body temperature regulation.
- Anxiolytics and sedatives: Reduce central nervous system activity, making it more difficult to perceive heat or thirst.
- Some antidiabetic medicines: Can increase the risk of dehydration or make the skin more sensitive to sunlight.
When these medicines are used alone or in combination, the risk of heatstroke increases substantially, especially on days when temperatures exceed 30°C.
What should you do if warning signs appear?
Recognising the early symptoms of heat intolerance is essential to prevent complications. Sometimes, the signs are subtle and may be mistaken for tiredness or feeling unwell.
Watch out for the following symptoms:
- Intense thirst or absence of sweating;
- Dizziness, weakness or confusion;
- Rapid pulse or low blood pressure;
- Nausea, hot and reddened skin;
- Dark-coloured or strongly-smelling urine.
In such cases, it is best to stop sun exposure immediately, find a cool place, drink water and, if necessary, consult a healthcare professional.
There is yet another risk: photosensitivity
As well as affecting the body’s thermal response, some medicines react directly with sunlight, causing skin lesions. This phenomenon is known as photosensitivity, and can manifest itself in two distinct ways:
- Phototoxicity: it occurs shortly after exposure to the sun and resembles a severe sunburn.
- Photoallergy: this develops later and resembles eczema; it may persist for several days.
Both reactions can leave permanent pigmented patches on the skin, even when using sun cream.
Which medicines are associated with these skin reactions?
The list is long and includes a number of commonly used medicines. Some examples:
- Antibiotics: doxycycline, ciprofloxacin, sulphonamides;
- Antidepressants: particularly tricyclics;
- Anti-inflammatory drugs (NSAIDs): such as ketoprofen and piroxicam;
- Diuretics: furosemide and hydrochlorothiazide;
- Antipsychotics: chlorpromazine and other phenothiazines;
- Medicines for the treatment of acne: isotretinoin;
- Herbal medicines: in particular St John’s wort;
- Chemotherapy and immunosuppressants: methotrexate, fluorouracil, amongst others.
It is important to note that not everyone who takes these medicines will experience a reaction, but the risk does exist and increases with prolonged exposure to the sun.
How can it be prevented?
The good news is that, with a few simple steps, you can stay protected without missing out on your summer days. Here are some effective strategies:
- Make sure you stay hydrated throughout the day, even if you don’t feel thirsty;
- Avoid exposure to the sun between 11 am and 5 pm;
- Use sun cream with an SPF of ≥30 and reapply it regularly;
- Opt for light clothing, wide-brimmed hats and sunglasses;
- Store medicines in a cool place, away from direct sunlight;
- Avoid leaving medication in the car or in very hot places;
- Ask your pharmacist for advice on the safest way to take your medication during the summer.
These measures are particularly important for people taking multiple medicines, for the most vulnerable, and for those who have previously experienced adverse reactions to heat or sunlight.
Prevent it before it happens
Many of the risks associated with heat and photosensitivity can be avoided.
At Correia Rosa Pharmacies, we help:
- To identify medicines that may cause sunstroke or photosensitivity;
- To be used in conjunction with specific sunscreens;
- Adopting practical strategies to stay hydrated and protect your skin.
It is common to associate the dangers of summer solely with excessive sun exposure or a lack of sun cream. However, the medication you take every day can silently increase the risk of serious problems, such as sunstroke or severe skin reactions.
By staying informed and seeking support from the teams at Correia Rosa Pharmacies, you can adapt to the heat safely and continue to enjoy the summer to the full.
Pop into one of the Correia Rosa pharmacies if you have any questions – you can count on us!
