Ketamine, a drug once popular in the club scene, is now psychiatrists' secret weapon against depression. Perhaps you've heard of it, either from celebrities' personal stories — such as former NBA player Lamar Odom — or through the many research studies currently underway to assess its effectiveness. What you may not know is how the drug works to relieve depression, and why it seems to help patients for whom all other treatments have failed. In that, you wouldn't be alone. Dr. Steven Levine, a board-certified psychiatrist who developed the protocol for the clinical use of ketamine in 2011, says scientists...