Taking echinacea supplements can reduce the risk of a cold by 58 percent and may also shorten the duration of a cold almost one and a half days, according to the paper, published on Sunday in the July issue of the journal The Lancet Infectious Diseases.

The study is a "meta-analysis" comparing the outcome of 14 published trials using echinacea.

One of the trials combined with echinacea with vitamin C, which showed the two together reduced the incidence of a cold by 86 percent.

The analysis was led by University of Connecticut pharmacist Craig Coleman.

Echinacea is a term for nine related daisy-like plant species that are native to North America and feature in the traditional medicine of the Sioux and other Plains Indians as remedies for infection, snakebites and rabies.

Other names for the plant are black sampson, Kansas snakeroot and purple coneflower.

Coleman's team said they had counted more than 800 products containing echinacea, which come in the form of tablets, extracts, fresh juice, tincture and tea.

Three of the nine species are commonly used (Echinacea purpurea, E. angustifolia and E. pallida), and different parts of the plant are used for different products.

The authors say it is still unclear how echinacea appears to stimulate the immune system against the cold virus.

Its three major ingredients are alkamides, chicoric acid and polysaccharides, but it is unclear whether these work by acting separately or together, or with the help of other constituents.

And the authors sound a word of caution, saying more work needs to be done on the plant's safety before doctors can recommend echinacea as a standard option for preventing or treating the common cold.