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WASHINGTON: Ketamine, a general anesthetic usually administered to children and pets, is also highly effective in low doses as an anti-depressant, according a recent study.

Researchers at Yale University wrote in 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.

"It's like a magic drug - one dose can work rapidly and last for seven to 10 days," said Ronald Duman, senior author of the study.

Anti-depressants slow to take effect

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% 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 mystery of ketamine's effect

"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.

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% of patients resistant to treatment with all other forms of anti-depressants were found to improve within hours after receiving ketamine.