Ketamine, a general anesthetic usually administered to children and pets but perhaps best known as a horse tranquilizer, is also highly effective in low doses as an anti-depressant, according a study published Thursday.

Researchers at Yale University wrote in the August 20 issue of the journal Science that unlike most anti-depressants on the market which can take weeks to take full effect ketamine can begin to counter depression in hours.

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“It’s like a magic drug — one dose can work rapidly and last for seven to 10 days,” said Ronald Duman, professor of psychiatry and pharmacology at Yale and senior author of the study.

The researchers noted that ketamine was tested as a rapid treatment for people with suicidal thoughts. Traditional anti-depressants can take several weeks to take effect, they noted.

About 40 percent of people suffering from depression do not respond to medication, and many others only respond after many months or years of trying different treatments.

The researchers found that ketamine improves depression-like behavior in rats by restoring connections between brain cells damaged by chronic stress.

“The pathway is the story. Understanding the mechanism underlying the anti-depressant effect of ketamine will allow us to attack the problem at a variety of possible sites within that pathway,” said George Aghajanian, another Yale scientist, who co-authored the study.

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Clinical use of ketamine has been limited because it has to be delivered intravenously under medical supervision and in some cases can cause short-term psychotic symptoms.

The National Institute of Mental Health found in a separate study that almost 70 percent of patients resistant to treatment with all other forms of anti-depressants were found to improve within hours after receiving ketamine.