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.