Detailed dosage guidelines and administration information for Allegra (fexofenadine hydrochloride). Includes dose adjustments, warnings and precautions. 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. 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. Fexofenadine (Allegra) is an antihistamine that treats and prevents allergy symptoms. It blocks histamine to reduce your symptoms. 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. 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.