Why does photodermatitis cause sun sensitivity?

Photodermatitis is an inflammatory response triggered or worsened by light. In one form, ultraviolet exposure changes a substance on or in the skin, and the immune system reacts to that altered substance. In another form, light directly activates an immune response in susceptible skin. Redness, itching, burning, small bumps or blistering may follow.

A medicine, fragrance, plant sap or skincare ingredient can sometimes make the reaction more likely. The rash may appear only on exposed areas, although some reactions extend beyond the edges of clothing.

Cover affected areas with clothing, seek shade and use a hat when outdoors. Ask a pharmacist or clinician whether a medicine or product could be contributing; do not stop a prescribed medicine without advice. Use bland, fragrance-free skincare and avoid applying new active products to inflamed skin.

A clinician may recommend a soothing treatment, anti-inflammatory medicine or testing to identify a trigger. The plan depends on the rash pattern, timing, exposures and medicines you use.

Arrange medical review if the rash is widespread, blistered, painful, recurrent or not settling after avoiding light. Seek urgent help for facial swelling, trouble breathing, faintness, eye involvement or extensive blistering. A fever or signs of infection, such as spreading warmth and pus, also need prompt assessment.

Could a medicine, fragrance, plant or skincare product be involved? Which wavelengths or activities should I avoid? What can I apply during a flare, and how should I prevent another reaction?