The psychedelic drug in "magic mushrooms" can quickly and effectively help treat anxiety and depression in cancer patients, an effect that might last for months, two small studies show.
The work released on Thursday is preliminary and experts say more definitive research must be done on the effects of the substance, called psilocybin.
But the record so far shows "very impressive results", said Dr Craig Blinderman, who directs the adult palliative care service at the Columbia University Medical Center/New York-Presbyterian Hospital.
He did not take part in the work.
Psilocybin, also called shrooms, purple passion and little smoke, comes from certain kinds of mushrooms.
It is illegal in the US, and if the federal government approves the treatment, it would be administered in clinics by specially trained staff, experts say.
Nobody should try it on their own, which would be risky, said the leaders of the two studies, Dr Stephen Ross of New York University and Roland Griffiths of Johns Hopkins University in Baltimore.
Psychedelic drugs have looked promising in the past for treating distress in cancer patients, studies stopped for years after a regulatory crackdown in the early 1970s.
Griffiths said it was not clear whether psilocybin would work outside cancer patients, although he suspected it might work in people facing other terminal conditions.
Plans were also under way to study it in depression that resists standard treatment, he said.
The new studies, published in the Journal of Psychotherapy, are small.
The NYU project, which also included psychotherapy, covered just 29 patients. The Hopkins study had 51.
In both studies, psilocybin treatment had more effect on anxiety and depression than a placebo did.
For example, by the day after treatment, about 80 per cent of the treated NYU patients no longer qualified as clinically anxious or depressed by standard measures.
That compared with about 30 per cent for the placebo group, which was a remarkably fast response, experts said, and it endured for the seven weeks of the comparison.
The studies took different approaches for formulating a placebo.
At NYU, patients were given niacin, which mimics some effects of psilocybin. At Hopkins, the placebo was a very low dose of psilocybin itself.
Researchers in both studies eventually gave full psilocybin treatment to the placebo groups and followed all the patients for about six months.
The beneficial effects appeared to persist during that period, but the evidence for that was less strong than for the shorter term because there was no longer any placebo comparison group.
