Diphenhydramine is an antihistamine commonly used to relieve symptoms of allergy, hay fever, and the common cold, such as sneezing, runny nose, and itchy eyes. It works by blocking the effects of histamine, a substance in the body that causes allergic symptoms. Diphenhydramine is also used as a nighttime sleep aid due to its sedative properties. It is available over-the-counter in various forms, including tablets, capsules, and liquid.