Ketamine is a medication primarily used for starting and maintaining anesthesia. It induces dissociative anesthesia, a trance-like state providing pain relief, sedation, and amnesia. The distinguishing features of ketamine anesthesia are preserved breathing and airway reflexes, stimulated heart function with increased blood pressure, and moderate bronchodilation. At lower, sub-anesthetic doses, ketamine is a promising agent for pain and treatment-resistant depression. However, the antidepressant action of a single administration of ketamine wanes with time, and the effects of repeated use have not been sufficiently studied.

Psychiatric side effects are frequent as well as raised blood pressure and nausea. Liver and urinary toxicity are common among regular users of high doses of ketamine for recreational purposes. Ketamine is an NMDA receptor antagonist, and that accounts for most of its actions except the antidepressive effect, the mechanism of which is a matter of much research and debate.

Ketamine was discovered in 1956 and approved for use in the United States in 1970. It was extensively used for surgical anesthesia in the Vietnam War due to its safety. Ketamine is also used as a recreational drug for its hallucinogenic and dissociative effects. Ketamine is on the World Health Organization's List of Essential Medicines. and is available as a generic medication.