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Volunteers rated the severity of their cold symptoms.

Echinacea, a herb widely used against symptoms of the common cold, works no better than a placebo, researchers have found in a systematic study.

The study compared three preparations containing extracts of echinacea, or purple coneflower, with a placebo in 399 volunteers who were exposed to a cold virus and then followed for five days.

The researchers tested extracts from a species of echnicaea that was recently endorsed by the World Health Organization as a treatment for cold.

Participants were asked to rate symptoms such as sneezing, blocked nasal passages, sore throat, cough and headache. The researchers also measured signs of inflammation.

Ronald Turner of the University of Virginia and his team concluded the extracts tested do not produce clinically significant effects on cold infections.

"Given the great variety of echinacea preparations, it will be difficult to provide conclusive evidence that echinacea has no role in the treatment of the common cold," the team wrote in the July 28 issue of the New England Journal of Medicine.

"Our study, however, adds to the accumulating evidence that suggests that the burden of proof should lie with those who advocate this treatment."

Many of the previous tests of echnicaea as a cold treatment showed conflicting results, but most were small studies with poor controls and industry sponsorship, noted Wallace Sampson in a journal commentary.

Turner's study was funded by the National Center for Complementary and Alternative Medicine, a U.S. government agency that pays for research on unconventional treatments that are popular.

"[R]esearch into implausible remedies rarely produces useful information," said Sampson, editor of the Scientific Review of Alternative Medicine and a emeritus clinical professor of medicine at Stanford University.

"Disproof rarely leads the supplement industry to reduce production or the public to decrease use. In fact, advocates often dismiss disproof."

Sampson calls on NCCAM to stop clinical trial research into alternative remedies with low plausibility of benefit.

Dr. Michael Murray, director of education for Factors Group of Nutritional Companies, said consumers shouldn't dismiss echinacea as a cold remedy based on the study alone. The treatment is most effective when three active components are combined in a specific ratio, he said in a statement.