Cetirizine is an antihistamine medicine that helps relieve the symptoms of allergies. It works by blocking the effects of histamine, a chemical produced by the body during an allergic reaction, and reduces unpleasant symptoms such as itchy, watery eyes, a running or blocked nose, sneezing, and skin rashes. Cetirizine is used to treat hay fever, conjunctivitis, eczema, hives, and other allergy symptoms. It is classified as a non-drowsy antihistamine, but some people may still feel quite sleepy after taking it. Common side effects of cetirizine include headaches, dry mouth, feeling sick, dizziness, and diarrhea. Cetirizine is generally safe for most people, but it is important to talk to a doctor or pharmacist before taking it if you have any allergies or medical conditions.