Ketamine, an anesthetic also used illegally as the club drug Special K, can relieve within hours, in contrast to current prescription medications that typically take weeks to kick in.

The drug eased depression in some study participants with just two hours, and its beneficial effect lasted for up to a week.

Ketamine is unlikely to become adopted as a treatment for patients, however, due to its hallucinogenic side effects -- such as seeing trails of light. Still, the findings suggest a new direction for developing faster-acting antidepressants, according to the researchers.

One of the main problems with current antidepressants is that they take a while -- sometimes weeks -- to begin working. This places some patients at high risk of suicide while they are waiting for their medication to kick in.

In the latest study, a research team led by Carlos Zarate, chief of the NIH Mood and Anxiety Disorders Research Unit, analyzed data from 17 volunteers. All were suffering from depression rated moderate to severe -- that is, they had a score of at least 18 on a standard depression test called the Hamilton Depression Rating Scale (HDRS) -- and all had failed to respond to two or more conventional drug treatments.

Initially, the participants received one dose of either ketamine or placebo. After a week, the system was reversed, with volunteers getting the opposite treatment -- except for those who still were demonstrating beneficial effects of their first dose.

For 12 of the 17 who received ketamine, symptoms of depression were reduced by 50 percent, based on the HDRS, within one day of receiving the drug.

Overall, compared to the patients receiving placebo, those treated with ketamine scored significantly lower on the HDRS when tested 110 minutes following the initial dose, and the effect lasted through seven days.

At day 1, the HDRS fell by up to 90 percent, the researchers reported, indicating a substantial reduction in depression. Twenty-nine percent reportedly had little or no evidence of depression on day 1 after the ketamine dose, while 71 percent were classified as "treatment responders." The treatment response held for at least a week for 35 percent of the participants.

Nine of the 17 patients had a 50% reduction in their depression within the first two hours of ketamine treatment, while only one individual who received placebo experienced the same effect.

Unlike most antidepressants, which boost the brain chemical serotonin, ketamine reduces the effects of glutamate, another neurotransmitter. This may explain why the drug acts faster, and may indicate a new direction for future research.

Zarate is currently investigating compounds derived from ketamine to see if any might have an antidepressant effect without causing hallucinations.