Ketamine is a dissociative anesthetic used medically for induction and maintenance of anesthesia. It is also used as a treatment for depression, a pain management tool, and sometimes as a recreational drug. Ketamine is a novel compound that was derived from phencyclidine in 1962, in pursuit of a safer anesthetic with fewer hallucinogenic effects.

At anesthetic doses, ketamine induces a state of "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, as with many antidepressants, the results of a single administration of ketamine wane with time. The long-term effects of repeated use are largely unknown, and are an area of active investigation.

Liver and urinary toxicity have been reported among regular users of high doses of ketamine for recreational purposes. Ketamine is an NMDA receptor pore blocker and that accounts for most of its actions, except the antidepressant effect, the mechanism of which is a matter of much research and debate.

Ketamine was first synthesized in 1962 and approved for use in the United States in 1970. It has been regularly used in veterinary medicine and was extensively used for surgical anaesthesia in the Vietnam War. Along with other psychotropic drugs, it is on the World Health Organization's List of Essential Medicines. It is available as a generic medication. When used as a recreational drug, it is found both in powder and liquid form, and is often referred to as "Special K" for its hallucinogenic and dissociative effects.