Taking the herbal product echinacea could reduce the chances of catching a cold by 58%, conclude authors of a Review published Online and in the July edition of The Lancet Infectious Diseases.

The Review also states echinacea could reduce the duration of colds by an average of 1·4 days. It was authored by Dr Craig Coleman, University of Connecticut School of Pharmacy, Hartford Hospital, Connecticut, USA, and colleagues. They conducted a meta-analysis (a study which combines the results of previous trials) of 14 studies into the use of echinacea to relieve/protect against catching a cold.

Only one of the 14 studies reviewed combined echinacea with vitamin C, which showed the two together reduced cold incidence by 86%. As a result the authors could not definitely conclude whether the two supplements combined are more effective than echinacea alone.

And the authors found that if echinacea was used in attempt to prevent "natural" catching of a cold, it reduced cold incidence by 65%; but if patients were directly inoculated with the cold-causing rhinovirus, echinacea use only reduced cold incidence by 35%. The authors say: "With over 200 viruses capable of causing the common cold, echinacea could have modest effect against rhinovirus but marked effects against other viruses."

Echinacea, a collection of nine related plant species indigenous to North America, is the most commonly used "nutraceutical" (herbal products, functional foods, animal based supplements). Echinacea angustifolia, Echinacea pallida, and Echinacea purpurea are the most common species recognised for their medicinal value. However the authors say: "The mechanism of action underlying the proposed immunostimulatory effects of echinacea remain unclear."

The Review found more than 800 products containing echinacea are available, and differing parts of the plant are used in different products (flower, stem, root). All three species of echinacea contain the same constituents, though in differing concentrations. It is thought that three of these (alkamides, chicoric acid, polysaccharides) could induce immunostimulation - though it is not clear whether it is one of these or if they are acting in combination either with each other, or other echinacea constituents.

The authors caution that more work needs to be done on the safety of echinacea, which was not covered in the Review.

They conclude: "An analysis of the current evidence in the literature suggests that echinacea has a benefit in decreasing the incidence and duration of the common cold; however, large-scale randomised prospective studies controlling for variables such as species, quality of preparation and dose of echinacea, method of cold induction, and objectivity of end points evaluated are needed before echinacea for the prevention or treatment of the common cold can become standard practice."

The Lancet Infectious Diseases