Description: Allegra is a powerful, nonsedating antihistamine (allergy medicine) that relieves seasonal allergy symptoms like sneezing, runny nose, and itchy, watery eyes. Allegra blocks the effect of histamine and relieves allergy symptoms such as sneezing, runny nose, and itchy, watery eyes. One dose of long lasting Allegra provides powerful relief from seasonal allergy symptoms for a full 24 hours.