Ketamine is a pharmaceutical perhaps most commonly used as an anesthetic (particularly in children and persons suffering from asthma) and painkiller, primarily in veterinary medicine, though it is also used in human medicine. It is also used as an emergency bronchodilator. It can cause hallucinations, and for this reason, it is also used recreationally. Typically called 'Special K' or 'K', its recreational use is illegal.
After House was shot in the episode No Reason, he requested ketamine in the hopes that it might eliminate his leg pain. He was right, but the effect was only temporary.