Detailed dosage guidelines and administration information for Allegra (fexofenadine hydrochloride). Includes dose adjustments, warnings and precautions. Overview: Fexofenadine is an antihistamine that is taken to help treat allergy symptoms, such as a runny nose, itchy or watery eyes, sneezing, or an itchy nose or throat. Common side effects may. While some other oral antihistamines may cause you to be drowsy, Allegra® won't. Fexofenadine, the active ingredient in Allegra ®, provides non-drowsy allergy relief with 0% brain interference, relieving allergy symptoms like sneezing, runny nose and itchy eyes. It is a common over-the-counter medication available under many brand names (including Allegra), as well as a generic drug. [11] Therapeutically, fexofenadine is a selective peripheral H 1 blocker. What is Allegra? Allegra (fexofenadine) is an antihistamine used to treat allergy symptoms, including sneezing, itching, watery eyes, and a runny nose in adults and children. Fexofenadine (Allegra) is an antihistamine that treats and prevents allergy symptoms. It blocks histamine to reduce your symptoms. Get fast-acting relief from seasonal allergies and symptoms like a runny nose, sneezing, itchy, watery eyes, and itchy nose or throat. Allegra ® kicks in within just one hour with 0% brain interference from drowsiness—that's right, it doesn't enter your brain and leave you feeling groggy.