Echinacea might be popular as a natural remedy for the common cold, but a large new study finds the supplement doesn't do much to help cold sufferers get better.

The research found only the tiniest benefit from the remedy: those taking echinacea reported that their cold symptoms lasted seven to 10 hours shorter than those not taking the pills. But the study's researchers say the effect was so small, they can't be sure it wasn't just due to random chance.

The study looked at 719 adults and children between the ages of 12 to 80 who lived in Wisconsin. Twelve per cent of the group were smokers; 64 per cent were female and 88 per cent were white.

The participants were randomly assigned to either not take anything for their cold, take a placebo pill, or take tablets of echinacea, which is made from the stems or roots of the purple coneflower. The tablets contained the equivalent of 675 mg of E. purpurea root and 600 mg of E. angustifolia root.

Some of the patients were told what they were taking and some were not told. Patients had to record their symptoms twice a day over the course of two weeks.

Overall, patients taking echinacea reported that the duration of their cold lasted about seven to 10 hours less than those not taking echinacea, regardless of whether they knew if they were taking the supplement. The colds of the echinacea takers were also slightly less severe, but only by about 10 per cent.

The investigators also measured levels of immune cell activity from nasal wash samples provided by the participants. They found the samples did not show any significant differences between those who took echinacea and those who didn't, suggesting there wasn't a major effect on the immune system.

While there appeared to be no side effects from the echinacea, the researchers say that for most people, the potential to get relief a few hours sooner probably isn't worth the cost of the supplement.

"Our previous research (72–74) suggests that few people -- no more than 1 in 4 -- would judge this level of benefit worthwhile, given the cost, inconvenience, and possible adverse effects," wrote the researchers led by Dr. Bruce Barrett, an associate professor of family medicine at the University of Wisconsin School of Medicine.

The findings are published in the Dec. 20 issue of Annals of Internal Medicine.

The study was funded by the National Center for Complementary and Alternative Medicine at the National Institutes of Health. The center's director, Dr. Josephine Briggs, said the conclusions of this study are consistent with a large body of previous clinical research. She says there are no further plans for more human research on echinacea.

"For now, the best advice for treating the common cold is to stay home, get plenty of rest, and drink lots of fluids," she said in a statement.