Photodermatitis (Sun-Triggered Dermatitis)

This page explains the two main mechanisms and how to find your trigger. For a broader overview of all sun rashes, including polymorphous light eruption, see dermatitis due to sun exposure.

Phototoxic vs photoallergic reactions

Phototoxic reactions are much more common. Some medicines can cause both types.

Common causes of photodermatitis

Medicines taken by mouth

Medicines linked to sun sensitivity include tetracycline antibiotics (especially doxycycline), fluoroquinolones, sulfonamides, thiazide diuretics, NSAIDs, amiodarone, some antifungals, St John's wort and some cancer drugs. See drug-induced photosensitivity for a fuller list and advice on continuing needed medicines.

Products on the skin

• Sunscreen filters such as oxybenzone (benzophenone-3) and octocrylene
• Fragrances in perfumes, aftershaves and cosmetics
• Topical NSAID gels
• Coal tar and some antiseptics

These usually cause photoallergic reactions, often confused with contact dermatitis, which happens without light.

Plants: phytophotodermatitis

Some plants contain chemicals called furocoumarins. When their juice gets on skin that is then exposed to sun, a phototoxic burn develops. Common sources are limes (squeezing them outdoors, the so-called "margarita burn"), lemons, celery, parsnips, figs, parsley, rue and giant hogweed.

The rash appears 1 to 2 days later as streaks, drips or handprint shapes, sometimes with blisters, and leaves brown marks that can last months. In children, the odd shapes are sometimes mistaken for bruises.

When the trigger is inside the body

Some people react to sunlight without any outside chemical, because of an underlying condition. These include polymorphous light eruption, solar urticaria, lupus, porphyria and pellagra (niacin deficiency).

How photodermatitis is diagnosed

Your doctor will look at the rash pattern. Clues to a light-triggered rash include involvement of the face, ears, back of the neck and hands, with sparing under the chin, behind the ears, under the nose and inside watch straps. They will ask about every medicine and supplement, skin products, plant contact and timing.

Further tests from a dermatologist include:

• Phototesting: small areas of skin are exposed to measured UVA, UVB and visible light to find abnormal sensitivity
• Photopatch testing: two identical sets of suspected allergens are applied to the back; one set is then exposed to UVA. A reaction only on the light-exposed side confirms photoallergy
• Blood tests for lupus antibodies or porphyrins when an internal cause is suspected

Treatment

• Stop or avoid the trigger. Do not stop a prescription medicine without talking to your doctor; a switch or extra protection may be possible.
• Cool compresses and soothing moisturizers for burning skin.
• Topical steroids for red, itchy or eczema-like areas. Severe cases may need a short course of oral steroids.
• Oral antihistamines for itch.
• Pain relief such as acetaminophen (paracetamol). If an NSAID was the trigger, avoid it.
• Leave blisters intact and keep them clean.
• Dark marks fade slowly; sun protection stops them deepening.

Preventing photodermatitis due to sun

• Use a broad-spectrum sunscreen with good UVA protection, SPF 30 or higher. Mineral sunscreens (zinc oxide, titanium dioxide) rarely cause photoallergy.
• Wear long sleeves, a wide-brimmed hat and UPF-rated clothing.
• Remember that UVA passes through car and home windows.
• Seek shade from 10 a.m. to 4 p.m.
• Wash hands after handling limes, celery or garden plants before going into the sun.
• Check medicine leaflets for sun warnings.

Frequently asked questions

What causes photodermatitis?

Most photodermatitis is caused by a chemical reacting with sunlight in the skin. The chemical can come from a medicine such as doxycycline or an NSAID, from a skin product such as a sunscreen or fragrance, or from plant juice such as lime. Less often, an underlying condition like lupus makes skin light-sensitive.

What is the difference between phototoxic and photoallergic reactions?

Phototoxic reactions can happen to anyone with enough of the chemical and sun. They appear within hours and look like a severe sunburn. Photoallergic reactions involve the immune system, need prior sensitization, appear 1 to 3 days after sun and look like itchy eczema.

How long does photodermatitis last?

Phototoxic reactions usually settle in several days to 2 weeks once the trigger and sun are avoided, though dark patches can last months. Photoallergic rashes often clear in 1 to 2 weeks with steroid creams, but they come back with each exposure until the allergen is identified and avoided.

What does phytophotodermatitis look like?

It appears as red, burning streaks, drips or finger-shaped marks, sometimes blistered, 1 to 2 days after plant juice such as lime touched the skin before sun exposure. As it heals, it leaves brown patches in the same unusual shapes that fade over weeks to months.

Can sunscreen cause photodermatitis?

Yes, occasionally. Some chemical filters, particularly oxybenzone and octocrylene, can cause photoallergic reactions. If you suspect your sunscreen, switch to a fragrance-free mineral sunscreen with zinc oxide or titanium dioxide, and ask a dermatologist about photopatch testing.