Echinacea, the popular herbal remedy for fighting the common cold, does not ward off runny noses, sore throats, or headaches, nor does it help speed recovery from cold symptoms, according to the results of a broad clinical trial to be reported today in the New England Journal of Medicine.

Taken with other recent studies that showed no benefit from echinacea, the new findings shift the burden of proof to proponents of herbal products to demonstrate that the plant has medicinal value, researchers said.

“We find no evidence that it actually does anything to common cold symptoms,” said Dr. Ronald Turner, a professor of pediatrics at the University of Virginia School of Medicine and the study’s lead author. “If that’s the reason you’re buying it, then you’re wasting your money.”

Echinacea enthusiasts said they do not think the results of the study merit such a clear-cut conclusion. They noted that Dr. Turner and his colleagues used only the root portion of one version of the plant and said the dosage given was too low to register any positive effect.

“This is a good contribution to the clinical literature, but it’s not the definitive study on echinacea,” said Mark Blumenthal, executive director of the American Botanical Council, a nonprofit group backed by herbal supplement makers whose logo includes a purple echinacea flower. “I just wish it had been a bigger study with bigger dosages.”

Echinacea, a member of the same plant family as sunflowers and daisies, was used for hundreds of years by more than a dozen American Indian tribes to treat snakebites, toothaches, coughs, and other ailments.

Western doctors began recommending it in the 19th century. It became popular in the America in the 1960s as consumers embraced herbal alternatives to traditional medicine. No less an authority than the World Health Organization recognized echinacea as a treatment for colds in 1999.

Americans spent $153 million on echinacea products last year, making it one of the five best-selling herbs in the country, according to the Nutrition Business Journal, an industry publication based in San Diego. It comes in capsules, tablets, tonics, powders, lozenges, tea bags, and even gummy vitamins for children. But spending has been declining steadily since 2001 as some users become disillusioned with the product, said editor Grant Ferrier.

“With a lot of herbal botanicals, including echinacea, there’s not a tangible effect,” Mr. Ferrier said. “It’s not like taking a pill for a headache. A lot of it goes on faith.”

Widespread consumer faith in echinacea prompted the National Center for Complementary and Alternative Medicine, a branch of the National Institutes of Health, to fund the two-year study. The goal was to pinpoint exactly how the herb attacks colds, said Dr. Stephen Straus, the center’s director.

Instead, the study concluded the plant served no such role.

“I would wish nothing more than for the echinacea study to be positive, but good science speaks for itself,” Dr. Straus said.

Dr. Turner and his colleagues tested three homemade preparations of echinacea, each designed to track the effect of a specific extract of the herb. All of the versions were derived from the root of an Echinacea angustifolia plant and contained the equivalent of 300 milligrams of echinacea per dose.

The researchers recruited 437 healthy volunteers and gave them a cold by squeezing droplets of the virus into their noses. Some of the volunteers took echinacea three times a day for one week before being infected. Others started taking it the day they were infected, and one group received a placebo throughout the experiment.

Once infected, the volunteers were sequestered for five days in hotel rooms, where their symptoms were carefully monitored.

At the end of the study, the researchers could not discern any difference between patients who took any form of echinacea and those who took the placebo.

“None of the preparations we used had any effect on either the rate of infection or the severity of illness,” Dr. Turner said.

Echinacea advocates insisted the study would have shown an effect if the dosage had been higher.