diazepam, tranquilizing drug used in the treatment of anxiety and as an aid in preoperative and postoperative sedation. Diazepam also is used to treat skeletal muscle spasms. It belongs to a group of chemically related compounds (including chlordiazepoxide) called benzodiazepines, the first of which was synthesized in 1933. Diazepam, known by several trade names, including Valium, was introduced in 1963. Side effects include drowsiness and muscular incoordination. A degree of physical dependence can result after prolonged use. The drug occurs as colourless crystals, and it is available for use in solution and in tablet form.