Used as an antihistamine to relieve symptoms of allergies, hay fever, and the common cold such as runny nose, sneezing, and itchy or watery eyes. It works by blocking the action 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 occasionally used in sleep aids.