n. a drug that inhibits the action of *histamine by blocking specific histamine receptors. Four histamine receptors have been identified (H1 to H4). H1-receptor antagonists are used for symptomatic relief of allergic conditions, such as hay fever, pruritus (itching), and urticaria (nettle rash). Many H1-receptor antagonists, e.g. *cyclizine and *promethazine, also have a strong *antiemetic activity and are used to prevent motion sickness. The most common side-effect of these drugs, especially the older antihistamines (e.g. *alimemazine, promethazine), is drowsiness and because of this they are sometimes used for sedation. Newer antihistamines, e.g. cetirizine, loratidine, mizolastine, are less sedating. Other side-effects include dizziness, blurred vision, tremors, digestive upsets, and lack of muscular coordination.
H2 receptors are mainly found in the stomach, where stimulation by histamine causes secretion of acid gastric juice. H2-receptor antagonists (e.g. *cimetidine, *nizatidine, *ranitidine, and *famotidine) block these receptors and reduce gastric acid secretion; they are used in the treatment of functional dyspepsia, *peptic ulcers, and *gastro-oesophageal reflux disease. H3- and H4-receptor antagonists have yet to find a clinical role.... antihistamine
Health Encyclopedia