Taking the herbal remedy ginseng reduces the risk of developing a cold, a Canadian study says.
Adults have two colds on average each year
The University of Alberta found just one in 10 people taking daily ginseng extract capsules had two colds or more, compared to a quarter of others.
Researchers also found ginseng reduced the severity of colds, the Canadian Medical Association Journal reported.
But UK doctors said that while many people did use the herbal remedy, the evidence was still anecdotal.
Adults have at least two colds a year on average, with children even more susceptible.
Ginseng, which is native to eastern Asia and North America, has long been associated with helping ward off flu and colds, but studies have never been able to establish a link.
In the study of 323 people, the Canadian team gave one group two capsules of a North American ginseng extract a day for four months during the winter and the second group a placebo, or dummy pill.
Just 10% of the ginseng group got two or more colds, compared to 23% of the others.
Symptoms and duration of cold were also much less severe - by about a third each - the researchers said.
It is thought ingredients in ginseng improve the immune system by stimulating immunoglobin - proteins that act as antibodies.
Lead researcher Dr Tapun Basu said the study had shown that the herbal remedy was "effective" in warding off colds.
And he added: "The safety of this formulation was also evident. It therefore appears to be an attractive alternative treatment for upper respiratory tract infections.
"However, further studies are required to assess its efficacy and safety."
Pregnant women, and people with diabetes and high blood pressure are all advised to avoid ginseng.
Dr Jim Kennedy, prescribing spokesman of the Royal College of GPs, said patients often asked about whether ginseng helped to protect against colds.
But he said: "We cannot advise people to use ginseng.
"While the supposed benefits of ginseng are widely known, there is still no proof it helps. The evidence is still anecdotal."