As their name suggests, tranquilizers are used to calm people in agitated states. Unlike barbiturates, which are minor tranquilizers used in the treatment of anxiety and neurotic disorders, a major tranquilizer, also known as an antipsychotic, is capable of alleviating symptoms of such major mental illnesses as schizophrenia, a psychotic disorder. Antipsychotics slow psychomotor activity in patients, providing them with emotional serenity and an indifference to events occurring around them.

Although the word tranquilizer suggests a tranquil, calm, or pleasant state, tranquilizers can bring on feelings of discomfort if ingested by nonpsychotic people. For this reason, these agents are seldom encountered as drugs of abuse.

The prototype of the largest class of antipsychotic drugs (the phenothiazines) was synthesized in the late 1800s, but its effects were not understood until a half-century later. The French first used tranquilizers in the early 1950s when they discovered that a derivative of...