Cetirizine is a second generation antihistamine that reduces the natural chemical histamine in the body. Histamine can produce symptoms of sneezing, itching, watery eyes, and runny nose. The dosage is based on your age, medical condition, and response to treatment. Do not increase your dose or take this medication more often than directed.Cetirizine is also used to treat an allergic reaction, itching and swelling caused by chronic urticaria (hives) and minimizes or eliminates the symptoms of perennial allergic rhinitis, seasonal allergic rhinitis, chronic idiopathic urticaria, allergic asthma, physical urticaria, and atopic dermatitis.Cetirizine can cause side effects that may impair your thinking or reactions. Be careful if you drive or do anything that requires you to be awake and alert.