A type of drug that slows down brain activity, which causes the muscles to relax and calms and soothes a person. CNS depressants are used to treat insomnia (trouble sleeping), anxiety, panic attacks, and seizures. They may also be used to relieve anxiety and tension before surgery. Examples of CNS depressants are benzodiazepines, barbiturates, and certain sleep medicines. CNS depressants are sometimes called sedatives or tranquilizers. Also called central nervous system depressant.