Ketamine has "shown promise" in rapidly treating the symptoms of depression and suicidal thoughts, a study has said.
Researchers in the US treated people with the drug via a nasal spray, and said they found a "significant" improvement in symptoms in the first 24 hours.
The study, by scientists at Janssen Research and Development in New Jersey, a Johnson & Johnson company, and the Yale School of Medicine, involved 68 people who were at imminent risk of suicide.
All were treated with anti-depressants throughout the trial, with half given esketamine - part of the ketamine molecule - twice a week for four weeks.
The other half were treated with a placebo.
Researchers said those treated with the esketamine saw a greater improvement in their symptoms after the first day, compared with those who had taken the placebo.
The effects had levelled out after 25 days, however.
According to the report, published in the American Journal of Psychiatry, the results showed the drug had fast-acting benefits for depressive symptoms, and could "bridge the gap" with antidepressants, which can often take four to six weeks to become fully effective.
However, the scientists also warned that more research was needed on the potential for the abuse and misuse of ketamine, which has a reputation as a party drug.
It is commonly used as an anaesthetic.
The nasal spray must now undergo a third phase of trials before it can be licensed for use.