Antihistamines are the most widely used medications to relieve or prevent symptoms of allergic rhinitis (hay fever)*. Some antihistamines may also be used to treat chronic urticaria (hives)*. These medications reduce symptoms by preventing the effects of histamine—a chemical substance produced by the body during an allergic reaction. Antihistamines are available as a liquid, tablet or nasal spray. Different antihistamines are available either by prescription or over-the-counter (OTC). Newer antihistamines prescribed by your allergist / immunologist are less likely to have side effects such as drowsiness, dry mouth, constipation or difficulty urinating.