Antihistamine, an tee HIHS tuh meen, is the name of a group of drugs used to treat symptoms of allergies. These symptoms include irritated eyes, runny nose, sneezing, and hives. Antihistamine drugs work by counteracting the effects of histamine, a substance found in many body cells. Excessive or inappropriate release of histamine into body fluids can cause allergies such as hay fever and contribute to dangerous reactions to insect bites and certain medications. Antihistamines are also used to treat motion sickness.