antihistamine /-his″təmin/ [Gk, anti + histos, tissue, amine (ammonia compound)] , any substance capable of reducing the physiological and pharmacological effects of histamine, including a wide variety of drugs that block histamine receptors. Many such drugs are readily available as over-the-counter medicines for the management of allergies. Toxicity resulting from the overuse of antihistamines and their accidental ingestion by children is common and sometimes fatal. These substances do not completely stop the release of histamine, and the ways in which they act on the central nervous system are not completely understood. The antihistamines are divided into histamine1 (H1) and histamine2 (H2) blockers, depending on the responses to histamine they prevent. H1-blocking drugs, such as alkylamines, ethanolamines, ethylenediamines, and piperazines, are effective in the symptomatic treatment of acute allergies. Second-generation H1 blockers, such as cetirizine, fexofenadine, and loratadine, cause less sedation. The H2-blocking drugs are effective in the control of gastric secretions and are often used in the treatment of gastroesophageal reflux. Antihistamines can both stimulate and depress the central nervous system. −antihistaminic, adj.
