tranquilizer, drug whose action calms the central nervous system, decreasing emotional agitation without impairing alertness. Tranquilizing drugs differ from hypnotic drugs such as barbiturates in that they do not act on the brain's cortical areas but rather on its lower portions, e.g., the hypothalamus. They have been found helpful in the treatment of tension and mental illness. Reserpine, which appeared on the market in 1952, was the first tranquilizer to be used in modern Western medicine. Other drugs used as tranquilizers include the phenothiazines, meprobamate, certain muscle relaxants and anticonvulsants, and lithium carbonate. See also psychopharmacology.

Drug used to reduce anxiety, fear, tension, agitation, and related disturbed mental states. Major tranquilizers (antipsychotic agents, or neuroleptics) are used to treat schizophrenia and other psychoses; phenothiazines, including chlorpromazine, are the best known. They are thought to block the activity of the neurotransmitter dopamine in the brain. Minor tranquilizers (antianxiety agents, or anxiolytics) are used to treat anxiety and tension; they are usually benzodiazepines, including diazepam (Valium) and chlordiazepoxide (Librium). They have a calming effect and reduce both physical and psychological effects of anxiety, fear, and stress by enhancing the action of the neurotransmitter gamma-aminobutyric acid (GABA) in the brain.

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