Depressants, also known as central nervous system depressants, or colloquially known as "downers", are a class of psychoactive drugs characterised by decreasing neurotransmission levels, decreasing the electrical activity of brain cells, or reducing arousal or stimulation in various areas of the brain. Commonly used depressants include alcohol, opioids, and benzodiazepines. Some specific depressants influence mood, either positively (e.g., opioids) or negatively (e.g., alcohol), but depressants often have no clear impact on mood (e.g., most anticonvulsants). In contrast, stimulants, or "uppers", increase mental alertness, making stimulants the opposite drug class from depressants. Antidepressants are defined by their effect on mood, not on general brain activity, so they form an orthogonal category of drugs.
Depressants are closely related to sedatives as a category of drugs, with significant overlap. The terms may sometimes be used interchangeably or may be used in somewhat different contexts. Nearly all commonly used depressants are addictive, and use of them carries the risk of death from respiratory depression, especially in opioids.