The federally funded study was what fans and foes of such substances say they have long needed rigorous, scientific testing. It found that patients who took an echinacea plant extract fared no better than those who took a dummy treatment.
"Our study ... adds to the accumulating evidence that suggests that the burden of proof should lie with those who advocate this treatment," wrote Dr. Ronald Turner of the University of Virginia School of Medicine, who led the study, which appeared in Thursday's New England Journal of Medicine.
Echinacea, or purple coneflower, is sold over-the-counter in pills, drops and lozenges. With reported annual sales of more than $300 million, Echinacea is one of the most popular medicinal herbs used by people to treat colds.
Several animal studies and small human trials have pointed to the possible benefit of the herb in preventing respiratory infections. However, one of largest studies involving 407 children in 2003, found that Echinacea failed to alleviate cold symptoms and even caused mild skin rashes in some cases.
In the newest experiment, researchers recruited 399 healthy patients who got one of three laboratory-made Echinacea plant extracts or a dummy preparation. The patients were then exposed to the cold virus and their symptoms were evaluated.
Scientists found no difference in infection rates between the groups who received the herb or placebo. About 90 percent in both groups wound up becoming infected. Symptoms like sneezing, runny noses and sore throat were also about the same, with more than half in both groups showing classic signs of a clinical cold.
The study was funded by the National Center for Complementary and Alternative Medicine, part of the federal National Institutes of Health. Turner, the lead researcher, has consulted for various antibiotic makers.
In an accompanying editorial, Dr. Wallace Sampson, an emeritus clinical professor of medicine at Stanford University School of Medicine, questioned why the government is wasting tax dollars on funding alternative medicine studies, some of which have turned out to be nothing but snake oil.
"Research into implausible remedies rarely produces useful information," wrote Sampson, who was not connected to the study.
CBS News Correspondent Elizabeth Kaledin reports that many critics of the study are suggesting that the 300 mg dose used was too small and that at higher doses Echinacea can actually help.
Mark Blumenthal, executive director of the American Botanical Council, an independent group that studies herbs, said people should not dismiss echinacea as a cold remedy. Blumenthal pointed out that the extract used in the latest study was prepared in the lab and not sold in stores. He also added that the herb might work better if higher doses were used.
"This is not a definitive trial on the efficacy of echinacea, nor should the results be generalized to echinacea preparations widely available," he said.