Allegra is a brand name for a medication that contains the active ingredient fexofenadine. It is an antihistamine used to relieve allergy symptoms such as sneezing, runny nose, and itchy eyes. Allegra works by blocking the action of histamine, a substance in the body that causes allergic reactions. It is commonly used to treat seasonal allergies and hives. Allegra is available in various forms, including tablets and liquid. It is typically taken once or twice a day, depending on the specific product and the severity of symptoms. Common side effects may include headache and drowsiness, but it is generally well-tolerated.