Used as an antihistamine to relieve symptoms of allergies, hay fever, and the common cold, such as sneezing, runny nose, and itching. It works by blocking the effects of histamine, a substance in the body that causes allergic symptoms. Often found in over-the-counter cold and allergy medications, sometimes combined with decongestants or pain relievers. May cause drowsiness, so it is also used in some sleep aids.