The Chinese herb ginseng could give exhausted cancer patients a physical and emotional boost, research suggests.
Ginseng is widely used by herbal practitioners
A US team at Rochester's Mayo Clinic found daily doses improved energy levels and emotional well-being, in a study of 282 patients.
They say that as studies show over half of cancer patients experience crippling fatigue, adding ginseng to cancer therapies is worth exploring.
Cancer experts urged caution until more work was carried out.
The work was presented to the American Society of Clinical Oncology.
Cancer fatigue is the most common side effect of cancer treatment and, according to Cancer Research UK, affects up to 90% of cancer patients.
Many people with cancer say it is the most disruptive side-effect of all, rendering them so tired they are unable to perform everyday tasks.
Ginseng has already been hailed as a remedy against colds and diabetes.
Scientists believe it works by acting as an "adaptogen" - a substance that helps the body overcome stress effects.
Since cancer patients can face high levels of stress, both physical and psychological, the Mayo team decided to test whether ginseng would be of benefit.
They enrolled 282 cancer patients and divided them at random into four groups - a control group, who received their normal cancer treatment and a dummy drug, plus three treatment groups who received one of three daily doses of ginseng (750mg, 1,000mg or 2,000mg) alongside their usual therapy.
After eight weeks, they surveyed the patients.
The group given the dummy drug reported no improvement, but the patients who had been taking the ginseng reported improvements in overall energy levels and experienced less fatigue-affected activity.
The ginseng groups said they felt better mentally, physically, spiritually and emotionally.
The improvements appeared to be dose related, with those on the highest dose reporting the greatest gains.
But the scientists stressed it would be premature to recommend ginseng supplements to cancer patients.
Lead researcher Dr Debra Barton explained: "Whilst the results were promising, we have more research to conduct."
Her team now plans to look at what dose is most appropriate.
Dr Barton said there were many different formulations of ginseng available on the market and that all might not work identically.
Josephine Querido, of Cancer Research UK, said: "It's too early to say whether using ginseng will help reduce tiredness in people with cancer. Further work may shed more light on this.
"Of the evidence currently available, exercise and support seem to be most effective at tackling tiredness in cancer patients."
She added: "If you're considering using complementary therapies, such as herbal supplements, you should always discuss this with your GP."