Cetirizine dihydrochloride is an antihistamine used to relieve symptoms such as watery eyes, runny nose, itching eyes/nose, and sneezing caused by hay fever or other allergies. It is also used to relieve itching and hives caused by some skin conditions. Cetirizine works by blocking the action of histamine, which is a substance produced by the body in response to an allergic reaction. Cetirizine is available in various forms, such as tablets, syrup, and chewable tablets, and is usually taken once a day. It is generally considered safe and effective for most people, but like any medication, it may cause side effects in some individuals. Common side effects may include drowsiness, dry mouth, headache, and fatigue.