Treatment of polymorphous light eruption usually isn't necessary because the rash typically goes away on its own within 10 days. If your symptoms are severe, your doctor may prescribe anti-itch medicine (a corticosteroid cream or pill). Treatment is also available to help prevent a rash.
Your doctor may suggest phototherapy to prevent seasonal episodes of polymorphous light eruption in people who have experienced disabling signs and symptoms. Phototherapy exposes your skin to small doses of UVA or UVB light, which helps your skin be less sensitive to light. Basically, it's a controlled version of the increased exposure you would experience over the course of the summer.
One type of light therapy called psoralen plus ultraviolet A (PUVA) combines UVA with a medicine called psoralen, which makes the skin more sensitive to this light. Short-term side effects of this therapy may include nausea, headache and itching.