Zyrtec is an over-the-counter medication used to relieve allergy symptoms such as sneezing, runny nose, and itchy eyes. Its active ingredient is cetirizine, an antihistamine that works by blocking the action of histamine, a substance in the body that causes allergic reactions. Zyrtec is commonly used for seasonal allergies, also known as hay fever, and can help alleviate symptoms caused by pet dander and dust mites. It is available in various forms, including tablets and liquid, and is typically taken once a day for effective relief.