Drug-induced photosensitivity.

Characteristics and diagnosis of photosensitivity are discussed, and drugs available in the United States that cause photosensitivity are identified. In phototoxic reactions, the drug absorbs energy from ultraviolet A (UVA) light and releases it into the skin, causing cellular damage. In photoallergic reactions, light may cause a structural change in a drug so that it acts as a hapten, possibly by binding to proteins in the skin. Once a hapten-protein complex is formed, Langerhans' cells residing in the epidermis can present the antigen to immunocompetent cells, causing hypersensitivity. Phototoxicity is much more common than photoallergy. Drugs that can cause phototoxic reactions include amiodarone, quinolones, and tetracyclines. Drugs that have been associated with photoallergic reactions include thiazides and benzocaine. Pharmacists should be aware of drugs that can cause photosensitivity and should counsel patients taking these drugs to avoid excessive exposure to sunlight.