antihistamine

n.

a drug that inhibits the action of histamine in the body by blocking either of two types of receptors for histamine, H₁ or H₂. 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 and promethazine, also have 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. acrivastine, cetirizine, loratidine, mizolastine) are less sedating. Other side-effects include dizziness, blurred vision, tremors, digestive upsets, and lack of muscular coordination.

H₂ receptors are found mainly 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 peptic ulcers and gastro-oesophageal reflux disease.