Zyrtec is an over-the-counter medication used to treat symptoms of allergies, such as sneezing, runny nose, itching, and watery eyes. It is an antihistamine that works by blocking the effects of histamine, a substance in the body that causes allergic symptoms. Zyrtec is available in various forms, including tablets, liquid gels, and syrup, and is typically taken once daily. It is generally well-tolerated, but some people may experience side effects such as drowsiness, dry mouth, or headache. It is important to follow the dosing instructions provided by a healthcare professional and to consult a doctor before starting any new medication.