Ketamine is an effective pain killer, however, it can be tricky to properly administer since over-dosing can easily happen. In recent years, ketamine has also been used for mood disorder, particularly as an immediate short-term treatment for severe depression to stabilize the patient. This does mean that it may be more readily available. It's meant for short-term usage since long-term usage is not recommended because a) there aren't many long-term studies and b) from what is known and from case studies, chronic use can result in altered enzymatic processes in the liver, so you'd need to regularly monitor them. Ketamine is also used at high enough concentrations to euthanize or put an animal "to sleep", so I would recommend asking your vet where to get prescribed ketamine.