New research suggests that the herb black cohosh, which is often used by women to ease the symptoms of menopause, may encourage the spread of breast cancer.

Vicki Davis and colleagues added black cohosh to the chow of female mice that were predisposed to breast cancer. Results should that the mice fed black cohosh were no more likely to develop breast cancer than mice not fed the herb. However, in mice that did develop breast cancer, the disease was significantly more likely to spread to the lungs in those fed chow containing black cohosh. The dose of black cohosh used in the study was equivalent to the amount typically recommended for women.

The authors concluded: “Additional studies will be needed to correlate these findings to women taking different black cohosh products at various times during breast cancer development; however, these results suggest caution for women using black cohosh, especially for extended periods of time. As metastatic progression is linked to patient survival, these data stress the importance of investigating how women's therapies influence all stages of mammary tumorigenesis, particularly for assessing their safety.”

Davis VL, Jayo MJ, Ho A, Kotlarczyk MP, Hardy ML, Foster WG, Hughes CL. Black Cohosh Increases Metastatic Mammary Cancer in Transgenic Mice Expressing c-erbB2. Cancer Res. 2008;68:8377-8383.