A sedative or tranquilliser (or tranquilizer, see English and American spelling differences) is a substance that induces sedation by reducing irritability or excitement.

At higher doses it may result in slurred speech, staggering gait, poor judgment, and slow, uncertain reflexes. Doses of sedatives such as benzodiazepines, when used as a hypnotic to induce sleep, tend to be higher than amounts used to relieve anxiety, whereas only low doses are needed to provide a peaceful effect.