The researchers concluded that the form of Echinacea used by their volunteers "provides no benefit for common cold symptoms in young, healthy adults."

The students who took the herbal supplement used a version that was a mixture of unrefined Echinacea purpurea herb (25 percent) and root (25 percent) and E. angustifolia root (50 percent).

They took six grams in one-gram doses per day for the first three days of the illness, and three grams each subsequent day of illness for a maximum of 10 days.

The Wisconsin team cautioned that other preparations of the herb might have different results, and that older individuals or people with compromised immune systems might benefit more from the herbal supplement.

Previous studies investigating involving Echinacea and the common cold have yielded somewhat more encouraging results, ranging from a 40 to 50 percent reduction in severity and duration of symptoms to more modest reductions of 10 to 30 percent in the most recent studies, according to background information in the study.

But virtually all of the studies have suffered from limitations, notably the lack of an objective way to validate self-reported symptoms.