Chris Stephens, 28, who has been battling depression all of his life, plays with his dogs at home in Concord, Calif., on Friday. After a dose of ketamine, Stephens says, "I actually wanted to do things. I wanted to live life." Lianne Milton for NPR hide caption

itoggle caption Lianne Milton for NPR

Ketamine has been used as an anesthetic for decades. It's also a widely popular but illegal club drug known as "Special K." When administered in low doses, patients report a rapid reduction in depression symptoms. Huw Golledge/flickr hide caption

itoggle caption Huw Golledge/flickr

The antidepressant Prozac selectively targets the chemical serotonin. Paul S. Howell/Getty Images hide caption

itoggle caption Paul S. Howell/Getty Images