Photoallergic reaction: An allergic reaction caused by drugs in which ultraviolet exposure changes the structure of the drug so that it is seen by the body's immune system as an invader. The allergic response causes inflammation of the skin in the sun-exposed areas. These usually resemble eczema and are generally long-lasting. Many drugs in this family are topical drugs. Individuals with photoallergic reactions may initially complain of itching (pruritus). This is then followed by redness and possibly swelling and eruption of the involved area. Common photoallergic drugs include some sunscreens, antimicrobials, painkillers, chemotherapy drugs, and fragrances.