Antihistamines are the most widely used medications for allergic diseases. They are used to treat symptoms of allergic rhinitis, or "hay fever," which affects more than 35 million people in the United States. Antihistamines can lessen your symptoms by preventing the effects of histamine—a chemical substance produced by the body during an allergic reaction.

Many different antihistamines are available, either by prescription or over-the-counter (OTC). Various forms include eye drops, nose sprays, liquids and pills, either as an antihistamine alone or mixed with other medications, especially decongestants, to relieve a larger range of symptoms.