antihistamine

an·ti·his·ta·mine .ant-i-'his-tə-.mēn, .an-.tī-, -mən adj tending to block or counteract the physiological action of histamine <human blood lacks \antihistamine activity>

antihistamine n any of various compounds that oppose the actions of histamine and are used esp. for treating allergic reactions (as hay fever), cold symptoms, and motion sickness

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n.

a drug that inhibits the action of histamine in the body by blocking the receptors for histamine, of which there are two types: H₁ and 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), for example acrivastine, azatadine, and chlorphenamine, 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. azatadine, brompheniramine, diphenhydramine, promethazine), is drowsiness and because of this they are sometimes used to promote sleep. Newer antihistamines (e.g. acrivastine, cetirizine (Zirtek), loratidine (Clarityn), terfenadine) 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 ulcer and gastro-oesophageal reflux disease.

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an·ti·his·ta·mine (an″te-) (an″ti-hisґtə-mēn) an agent that counteracts the action of histamine. While the term can broadly include any agent that blocks any histamine receptor, in practice it is usually used to denote those blocking H₁ receptors (H₁ receptor antagonists), which are the drugs conventionally used to treat allergic reactions and are also components of many cough and cold preparations. Agents blocking H₂ receptors are usually called histamine H₂ receptor antagonists, and include the agents used to inhibit gastric secretion in peptic ulcer. See also histamine.