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 (seeallergy) or inflammation. Some antihistamines also prevent motion sickness and vertigo. Drowsiness is a frequent side effect. H antihistamines, which bind to the second receptor type, are used to control gastric-acid secretion (seestomach) and treat peptic ulcers.