Cetirizine is an antihistamine used to relieve allergy symptoms such as watery eyes, runny nose, itching eyes/nose, sneezing, hives, and itching. It works by blocking a certain natural substance (histamine) that your body makes during an allergic reaction. Cetirizine does not prevent hives or prevent/treat a serious allergic reaction (such as anaphylaxis). If your doctor has prescribed epinephrine to treat allergic reactions, always carry your epinephrine injector with you. Do not use cetirizine in place of your epinephrine.