The trial looked at 68 people at imminent risk of suicide.
All patients were treated with a stay in hospital and anti-depressants.
In addition, half were given ketamine in the form of esketamine (part of the ketamine molecule) in a nasal spray and half were given a placebo.
The study found those using esketamine had a much greater improvement in depression symptoms at all points over the first four weeks of treatment.
However, at 25 days the effects had levelled out.
The study’s authors suggest it could offer an effective rapid treatment for people severely depressed and at imminent risk of suicide and could help in the initial stages of treatment, as most anti-depressants take four to six weeks to become fully effective.
The nasal spray is now undergoing phase three trials before it can be licensed for treatment.