Some common side effects of Allegra include vomiting, diarrhea, dizziness, headache and drowsiness, according to Drugs.com. Rare side effects of Allegra include nervousness, rash, problems sleeping and night terrors. Chest tightness, difficulty breathing, and redness of the face, neck and arms are rare side effects that require immediate medical care.
Allegra sometimes causes cough, pain during menstruation, and generalized pain in the arms, legs or back, reports MedlinePlus. Patients should tell a doctor about any side effects that are persistent or bothersome. Allegra is an antihistamine that treats seasonal allergy symptoms, including runny nose, sneezing, itchy or watery eyes, and itching of the nose, throat or roof of the mouth. The medication also treats certain skin rashes and hives.