Ketamine, a farm-yard tranquilliser gaining notoriety for its popularity among recreational drugs users, may have an unexpected upside.

A single intravenous infusion of the general anaesthetic agent can relieve symptoms of depression within two hours and remain effective for up to one week, according to a report in the August issue of Archives of General Psychiatry.

US researchers say that ketamine directly targets a different brain pathway compared to traditional anti-depressants – and the finding could lead to a new class of treatments that act much faster than existing drugs.

Most currently available medications do not begin to relieve the symptoms of depression for several weeks. During this lag period, patients are at risk for self-harm and even suicide, especially within the first nine days of treatment.

“Drugs that have rapid onset of antidepressant effects within hours or even a few days and that are sustained would therefore have an enormous impact on public health,” said Dr Carlos Zarate of the National Institute of Mental Health in Bethesda.

His team studied the effects of intravenous ketamine in 12 women and six men (average age 46.7 years) with treatment-resistant depression. After a two-week drug-free period, participants received two intravenous infusions one week apart; one with ketamine (0.5 milligrams per kilogram of body weight) and the other with placebo.

Within 110 minutes after the infusion, patients who received ketamine rather than placebo showed significant improvement. The effect lasted until the following week. On the day after treatment, 71% of the 17 participants who received the ketamine responded to the drug and 29% met criteria for remission from depression, compared with none who received the placebo.

“To our knowledge, there has never been a report of any other drug or somatic treatment that results in such a dramatic rapid and prolonged response within a single administration,” Dr Zarate said. No serious adverse events occurred during the study.