When they feel a cold coming on, many people reach for what they believe will be a safe preventative — a tea or capsule containing the herb echinacea. But does it really work? Worse, is it bad for you?

Recently, the first study to examine the effects of a botanical supplement on bacteria in the gastrointestinal tract suggested that people may be risking some unwelcome side effects with this common herbal remedy.

Although various botanical extracts have been proven to have antimicrobial effects, previous to this research there had been no published information about their effect on the human gastrointestinal tract, home to many varieties of beneficial bacteria.

Research conducted by Jerald C. Foote, assistant professor of food and human nutrition at the University of Arkansas, and graduate student Laura Hill yielded some surprising results.

The researchers worked with 15 healthy adults who took daily doses for 10 days of a supplement containing Echinacea purpurea. The participants recorded any adverse effects, which proved to be minor. Stool specimens were collected and tested prior to using echinacea and again 10 days and 17 or 18 days following the supplementation.

Although the researchers had expected to see no dramatic effects from the echinacea, they found that gastrointestinal bacterial counts actually increased for three groups of organisms during supplementation.

Concentrations of all types of aerobic bacteria increased, as did those of the anaerobic Bacteroides in general and Bacteroides fragilis in particular. Anaerobic bacteria — those that grow where there is no oxygen — comprise the majority of the bacteria in the human colon. While Bacteroides are an important feature in the normal life of the colon, under certain conditions they also can cause illness, especially strains of B. fragilis.

This result was somewhat alarming. Foote and Hill point out that increased Bacteroides concentrations have been associated with high risk for colon cancer. Studies have also shown that a specific strain of B. fragilis may contribute to inflammatory bowel disease and diarrhea.

Writing in the Journal of Clinical Pharmacy and Therapeutics, the UA researchers noted that their study showed a significant increase in bacteria after only 10 days. They point out that the German Commission E, a body that sets guidelines for the use of herbal products in Germany, advises that echinacea can be used for up to eight weeks. In light of their research, Foote and Hill caution that "prolonged consumption could prove deleterious" to bacteria in the gastrointestinal tract, particularly for those who are ill.