A depressant, or central depressant, is a drug that lowers neurotransmission levels, which is to depress or reduce arousal or stimulation in various areas of the brain. Depressants are also colloquially referred to as "downers" as they lower the level of arousal when taken. Stimulants, or "uppers," increase mental or physical function, hence the opposite drug class from depressants is stimulants, not antidepressants.

Depressants are widely used throughout the world as prescription medicines and illicit substances. Alcohol is a very prominent depressant. Alcohol can be, and is more likely to be, a large problem among teenagers and young adults. When depressants are used, effects often include ataxia, anxiolysis, pain relief, sedation or somnolence, cognitive or memory impairment, as well as in some instances, euphoria, dissociation, muscle relaxation, lowered blood pressure or heart rate, respiratory depression, and anticonvulsant effects. Depressants also act to produce anesthesia. Cannabis may sometimes be considered a depressant due to one of its components, cannabidiol. The latter is known to treat insomnia, anxiety, and muscle spasms similar to other depressive drugs. However, while tetrahydrocannabinol, another component, may slow brain function to a small degree while reducing reaction to stimuli, it is generally considered a stimulant and main psychoactive agent that can sometimes cause anxiety, panic, and psychosis instead. Other depressants can include drugs like Xanax (a benzodiazepine) and a number of opioids. Gabapentinoids like gabapentin and baclofen are depressants and have anticonvulsant, and anxiolytic effects. Most anticonvulsants like Lamotrigine and Phenytoin are depressants. Carbamates (Felbamate Carisoprodol) are depressants that are similar to barbiturates.

Depressants exert their effects through a number of different pharmacological mechanisms, the most prominent of which include facilitation of GABA and inhibition of glutamatergic or monoaminergic activity. Other examples are chemicals that modify the electrical signaling inside the body, the most prominent of which are bromides and channel blockers.