Cetirizine hydrochloride is a second-generation antihistamine used to treat allergies, such as hay fever, allergic rhinitis, and hives. It works by blocking the action of histamine, which is released during an allergic reaction, thereby reducing symptoms like sneezing, itching, runny nose, and watery eyes. Cetirizine hydrochloride is available in various forms, including tablets, capsules, and syrup, and is sold under several brand names, such as Zyrtec, Reactine, and Cetiriz. It is usually taken once a day and is generally considered safe for most people. However, it may cause some side effects, such as drowsiness, dry mouth, and headache, in some individuals.