Despite encouraging signs in trials, prescribing the anaesthetic ketamine for depression is premature, an expert says.

Doctors shouldn't jump the gun in prescribing patients the drug ketamine to treat depression, warns an expert.

The anaesthetic and analgesic drug, also used as a horse tranquilliser or party drug, has shown promising signs in single-dose clinical trials.

But the efficacy and safety of repeated dosing has not yet been tested, Professor Colleen Loo, from the University of NSW and the Black Institute in Sydney, writes in the Medical Journal of Australia.

"Some clinics in Australia and overseas have begun offering a course of ketamine treatments to patients with depression.

"However, this practice is premature, given that the efficacy and safety of this treatment approach has yet to be tested in controlled trials."

A single-dose of ketamine takes effect within 24 hours of administration, unlike other antidepressants which can take several weeks.

But its antidepressant effects typically only last several days after a single treatment.

Uncontrolled data from recreational users of ketamine suggest risks associated with longer-term use may include liver damage, bladder dysfunction, cognitive impairment and possible addiction, she wrote.

Last month, the federal government announced funding of $2 million for a new trial to determine whether the drug can be used for treatment-resistant depression.