New Zealand and Australia are to conduct the world's largest trial yet to find out whether ketamine can cure depression.
Two hundred patients who've failed to get better using existing treatments will take part - some getting a placebo, the rest twice-weekly ketamine treatments.
Previous trials have shown ketamine "produces rapid antidepressant effects within hours", says research leader UNSW Professor Colleen Loo, but the long-term effects aren't yet known.
"This trial will allow us to examine the effects of repeated dosing and whether the positive effects of ketamine on an individual's depression can be sustained over a longer period."
The patients won't know if they're getting a placebo or ketamine, nor will the doctors administering the treatment.
After the trial ends, patients who are responding well will be given the chance to keep taking ketamine.
Ketamine's potential as an antidepressant was discovered in 2006. It's been used for decades as a pain reliever and animal tranquiliser, as well as finding popularity on the street as 'Special K'.
With evidence of the drug's effectiveness as a fast-working antidepressant growing, some doctors in Australia have already started giving it to their patients, off-label.
"This practice is premature and irresponsible, given that the effectiveness and safety of this treatment approach involving repeated dosing has yet to be tested in controlled trials," says Prof Loo.
Prof Colleen Loo (supplied)
Existing antidepressant drugs can take weeks to have an effect, have numerous side-effects and don't always work, forcing patients to undergo lengthy trial and error periods while they find one that does.
"We will be working very closely with clinical pharmacologists during this trial to understand the specific dosage required for each individual and the likely effects it will have," says Prof Loo.
The New Zealand part of the research will be conducted at the University of Otago.
Ketamine is currently listed as a class C drug under the Misuse of Drugs Act 1975, the same category as marijuana.