Medicine-related photosensitivity: when sunlight and a medicine react

A medicine that reaches the skin through the bloodstream, or a cream applied directly, can absorb ultraviolet energy. That energy can then injure skin cells directly or trigger an immune response. Neither process needs the sun to be strong, and some reactions occur through a window or on a cloudy day.

Clinicians separate two types. Phototoxic reactions are not immune-mediated and relate to how much of the medicine is present and how much light reaches the skin, so in principle they can affect anyone taking enough of a photosensitising drug. Photoallergic reactions need an earlier sensitising exposure, usually over about a week or more, and then flare on re-exposure, which is why they affect only a minority of people.

In practice the two overlap, and a clinician decides from the story, the distribution and the medicine list rather than from the rash alone.

DermNet lists tetracyclines such as doxycycline, fluoroquinolone antibiotics, anti-inflammatories such as ibuprofen and naproxen, amiodarone, furosemide and oral retinoids among phototoxic agents. Topical anti-inflammatory gels, some fragrances, phenothiazines and certain sunscreen ingredients are examples linked with photoallergy. The NHS page for doxycycline lists skin that is more sensitive to the sun as a side effect, with rash, itching and redness.

The list is not exhaustive, and a patient information leaflet is the quickest place to see whether a new medicine carries a sun warning. Herbal products such as St John's wort are also associated with light sensitivity, so it is worth including supplements when you review your medicines with a pharmacist.

Most diagnoses rest on timing: a new medicine, then a rash on exposed areas of the face, backs of the hands, forearms and the V of the neck, with the area under a watch strap or shirt collar spared. Photo testing and photopatch testing in a specialist dermatology unit can reproduce a reaction under controlled light, and are used when the cause is unclear or a person must stay on a medicine that is under suspicion. A skin biopsy is rarely needed.

Other conditions can mimic the pattern, including polymorphic light eruption, lupus and sunburn from simple overexposure, so a rash that keeps returning in summer deserves a proper review even if no medicine seems responsible.

The main treatment is to identify the medicine and, where it is safe to do so, replace it or change when it is taken. The decision rests with the prescriber, because some of these medicines treat serious conditions. In the meantime, strict photoprotection is advised: broad-spectrum high-SPF sunscreen, protective clothing, a wide-brimmed hat and avoiding the middle of the day.

Cool compresses, simple moisturisers and, where needed, topical or short courses of oral steroids can calm inflammation. The NHS sunburn page recommends cooling the skin, using aftersun or unperfumed moisturiser, drinking plenty of water and avoiding petroleum jelly, ice packs and bursting blisters. It also recommends 111 or an urgent GP appointment for blistering, swelling, fever or chills, dizziness or sunburn in babies and young children.

• Read the leaflet for any sun warning when a new medicine is started.

• Apply sunscreen even on cloudy days and reapply as directed on the product.

• Remember that window glass blocks some, not all, of the light that triggers reactions.

• Tell the dermatologist about every cream, tablet and herbal product used.

Dark patches can remain after a phototoxic reaction and fade slowly. Photoallergic reactions are occasionally persistent. The NHS notes that sunburn can increase the risk of skin cancer, which is a further reason to avoid repeated burns while taking a photosensitising medicine over many months.