A new study has found that for people seeking a natural treatment for the common cold, some preparations containing the plant Echinacea work better than nothing, yet "evidence is weak".

The evidence review revealed no significant reductions in preventing illness, but didn't rule out "small preventive effects."

The six authors conducted reviews on this subject in 1998, 2006 and 2008 and wanted to do an update to include several new trials conducted since then.

"We've been doing this for so long and are very familiar with past research- which has been mixed from the very beginning," author Bruce Barrett , M.D., Ph.D. in the department of family medicine at the University of Wisconsin in Madison, said.

The research team reviewed 24 randomized controlled trials to determine whether Echinacea was a safe and effective cold prevention and treatment.

Trials included 4631 participants and 33 preparations, along with placebo. Echinacea products studied in these trials varied widely according to characteristics of three different plant species, the part of the plant used and method of manufacturing.

Barrett added that "it looks like taking Echinacea may reduce the incidence of colds. For those who take it as a treatment, some of the trials report real effects-but many do not. Bottom line: Echinacea may have small preventive or treatment effects, but the evidence is mixed."

The study was published in the journal Cochrane Library.