On the seventh floor of a building overlooking the Federal Reserve Bank in lower Manhattan, two medical clinics share an office. One is run by a podiatrist who’s outfitted the waiting room with educational materials on foot problems such as hammer toes and bunions. The other clinic doesn’t have pamphlets on display and offers a much less conventional service: For the advertised price of $525, severely depressed and suicidal patients can get a 45-minute intravenous infusion of ketamine—better known as the illicit party drug Special K. ... Patients receive a low dose of the drug: about one-tenth of what recreational abusers of ketamine take or about one-fifth of what might be used as a general anesthetic. ... During the infusions, which are gradual rather than all at once, patients often experience strange sensations, such as seeing colors and patterns when they close their eyes. ... The U.S. Food and Drug Administration hasn’t approved ketamine for the treatment of mood disorders, but dozens of medical studies show that it can quickly alleviate severe depression.