An antihistamine is a drug that is used to treat allergies. a drug that is used to treat an allergy (=an unpleasant reaction to particular foods, substances etc). Synthetic drug that counteracts the effects of released histamine in the body. Antihistamines compete with histamine at one of the three types of histamine receptors, preventing allergic attacks (see allergy) or inflammation. Some antihistamines also prevent motion sickness and vertigo. Drowsiness is a frequent side effect. H2 antihistamines, which bind to the second receptor type, are used to control gastric-acid secretion (see stomach) and treat peptic ulcers
a chemical that blocks the affect of histamine in a susceptible tissues Histamine is released by immune cells during an allergic reaction and also during infection with viruses which cause the common cold The interaction of histamine with the mucus membranes of the eyes and nose results in "watery eyes" and the "runny nose" often accompanying allergies and colds Antihistamines can alleviate such symptoms