The herbal remedy echinacea is not effective in treating children's colds, researchers have found.
A wide range of herbal medicines are used
A study compared the treatment with a dummy pill and found no difference in how long the children were ill for.
Over 500 children were studied by doctors at the University of Washington in Seattle.
But herbalists said the study contradicted other research which showed that echinacea was "powerful and effective".
The study looked at upper respiratory tract infections in children.
Most children have between six and eight such infections each year, each of which lasts around a week.
Children may be given drugs such as decongestants, antihistamines, or cough suppressants, but there is little evidence that these are effective in those under 12.
The researchers studied children aged between two and 11.
Half the children studied were given Echinacea purpurea, with the rest given a dummy pill, or placebo, for up to three infections over a four-month period.
The echinacea or placebo was started at the onset of symptoms and continued for a maximum of 10 days.
In total, 407 children suffered 707 infections during the period of the study. Just over half occurred in children who were treated with the placebo.
Dr James Taylor, who led the study, said: "There was no difference in duration between upper respiratory tract infections treated with echinacea or placebo.
"There was also no difference in the overall estimate of severity of upper respiratory tract infection symptoms between the two treatment groups."
There was also no difference between the groups in terms of the day on which the children's symptoms peaked, how many days they had a fever and how severe parents thought infections were.
However, rashes occurred in 7.1% of the upper respiratory tract infections treated with echinacea and 2.7% percent of those treated with the dummy pill.
Dr Taylor added: "Given its lack of documented efficacy and an increased risk for the development of rash, our results do not support the use of echinacea for treatment of upper respiratory tract infections in children two to 11 years old.
"Further studies using different echinacea formulations, doses, and dosing frequencies are needed to delineate any possible role for this herb in treating colds in young patients,"
But Trudy Norris, president of the National Institute of Medical Herbalists, said: "This research contradicts many other studies that show the beneficial effect of taking echinacea."
She added: "I know that a lot of people do self prescribe and use echinacea but it's preferable to see a professional rather than second guessing the right herbs to use.
"Medical herbalists seldom recommend single herb treatments, or supplements as a 'one pill fits all' solution.
"Qualified medical herbalists treat patients individually, with individually formulated herbal medicine, which may contain a number of active herbal ingredients. If used properly, echinacea is a powerful and effective herb."
The research is published in the Journal of the American Medical Association (JAMA).