antihistamine

views updated

antihistamine (anti-hist-ă-meen) n. a drug that inhibits the action of histamine in the body by blocking the H1 or H2 receptors for histamine. When stimulated by histamine, H1 receptors may produce such allergic reactions as hay fever, pruritus (itching), and urticaria (nettle rash). Antihistamines that block H1 receptors (H1-receptor antagonists) are used to relieve these conditions. Many H1-receptor antagonists (e.g. cyclizine, promethazine) also have strong antiemetic activity and are used to prevent motion sickness. The most common side-effect of these drugs, especially the older ones (e.g. alimemazine, promethazine), is drowsiness and because of this they are sometimes used to promote sleep. Newer antihistamines (e.g. acrivastine) are less sedating. H2 receptors are found mainly in the stomach, where stimulation by histamine causes secretion of acid gastric juice. H2-receptor antagonists (e.g. cimetidine, famotidine, nizatidine, ranitidine) block these receptors and so reduce gastric acid secretion; they are used in the treatment of peptic ulcers and gastro-oesophageal reflux disease.

More From encyclopedia.com