Allegra 180 mg Tablet 10's is primarily used to treat various kinds of allergies. An allergy is an immune system response to foreign elements typically not harmful to your body. Allergic condition varies from person to person.
Allegra 180 mg Tablet 10's contains fexofenadine, which blocks the effects of a chemical messenger known as histamine, which is naturally involved in allergic reactions. 
In some cases, you may experience side effects like headaches, nausea, or dizziness. Most of these side effects of Allegra 180 mg Tablet 10's do not require medical attention and gradually resolve over time. However, if the side effects are persistent, reach out to your doctor.