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.