Echinacea, a wild flower (also known as the purple coneflower), is sold in capsule form in drug and retail stores. Dried echinacea root has been used in homemade remedies such as teas, dried herb and liquid extracts.
Researchers from the University of Wisconsin-Madison School of Medicine and Public Health tested more than 700 patients, aged 12 to 80 years - all with an early symptom of cold, to see whether echinacea was effective in curing their symptoms.
One group received no pills, a second group received what they knew was echinacea and a third group was given either echinacea or a placebo, but they did not know which.
Participants recorded their symptoms twice a day for the duration of the cold, up to two weeks.
Bruce Barrett, the lead researcher and an associate professor of family medicine, found that the patients receiving echinacea saw the duration of their cold reduced by seven to 10 hours - not considered a significant decrease.
"Trends were in the direction of benefit, amounting to an average half-day reduction in the duration of a weeklong cold or an approximate 10 percent reduction in overall severity," he said.
"However, this dose regimen did not make a large impact on the course of the common cold, compared either to blinded placebo or to no pills," he added.
Barrett said a larger trial involving people who have found echinacea useful may help provide more answers.
He added that there were no side effects seen, so there is no reason that cold sufferers should stop using echinacea if they think it helps them.
"Adults who have found echinacea to be beneficial should not discontinue use based on the results of this trial, as there are no proven effective treatments and no side effects were seen," said Barrett.
The study is published in this month's Annals of Internal Medicine.