Ketamine is an anesthetic drug which has been used to provide anesthesia for surgery and to treat depression since the 1950s. When used for surgery it is given in high enough dose to cause unconsciousness, profound amnesia and pain control. When treating depression, it is given in a very low dose over a period of about 40 minutes, by intravenous infusion. The patient remains conscious and able to communicate. It was first noticed that ketamine might be useful to treat depression in the 1950s, but has only been in the last few years that its effectiveness has been widely recognized. There is now a growing consensus on the appropriate protocol for use of ketamine when treating depression. Many studies have been done and they consistently show effectiveness.