Hydroxyzine is an antihistamine with anticholinergic (drying) and sedative properties that is used to treat allergic reactions. The body releases histamine during several types of allergic reactions and, to a lesser extent, during some viral infections, such as the common cold. When histamine binds to its receptors on cells, it causes the cells to release chemical messengers that lead to sneezing, itching, and increased mucus production. Antihistamines, like histamine, bind to the histamine receptors. When they bind to the receptors, however, they do not stimulate the cells to release chemical messengers. In addition, they prevent histamine from binding and stimulating the cells. Hydroxyzine itself has no histamine-like activity. After ingestion, it is converted to its active form. The active form of hydroxyzine is a drug called cetirizine (Zyrtec). Although, both hydroxyzine and cetirizine act as antihistamines, hydroxyzine causes more sedation than cetirizine. The FDA approved hydroxyzine in 1956.