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. H 2 antihistamines, which bind to the second receptor type, are used to control gastric-acid secretion (see stomach ) and treat peptic ulcer s.