Among common health problems, depression was linked to the highest increase in annual spending by employers' on workers' health care. iStockphoto.com hide caption

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A rat neuron before (top) and after (bottom) ketamine treatment. The increased number of orange nodes are restored connections in the rat's brain. Ronald Duman/Yale University hide caption

itoggle caption Ronald Duman/Yale University

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