In the year 2000, psychiatry experienced one of the most important breakthroughs in decades: researchers showed that ketamine, a widely used anesthetic, could produce a powerful antidepressant effect within hours. For patients suffering from severe depression, this was something radically new. Traditional antidepressants take weeks or months to work, and many patients never respond at all. Ketamine was capable of relieving depressive symptoms in the majority of these patients rapidly, even when they did not respond to several conventional antidepressants.