Tranquilizer: In pharmacology, a drug that calms and relieves anxiety. The first tranquilizer, chlordiazepoxidehydrochloride (brand name: Librium), received FDA approval in 1960. Tranquilizers range in potency from mild to major, with increasing levels of drowsiness occurring as potency increases. They are prescribed for a wide variety of conditions but are used primarily to treat anxiety and insomnia. Most tranquilizers are potentially addictive, particularly those in the benzodiazepine family.