That's according to a study published recently in Cancer Research.
In the study, female mice with a genetic predisposition for breast cancer ate either normal chow or chow laced with black cohosh throughout their adult lives.
That's "concerning," researcher Vicki Davis, PhD, tells WebMD.
Davis, an assistant professor of pharmacology-toxicology at Duquesne University in Pittsburgh, notes that the black cohosh dose used in the study is equivalent to the amount typically recommended for women, but it remains to be seen if the results apply to women, to other types of cancer, and to cancer's spread to organs other than the lungs.
It's also not yet clear if the timing of exposure to black cohosh makes a difference. "Whether we needed black cohosh to be present as the tumors were developing in order for them to be more aggressive, we don't know that," Davis says.
Supplements Industry Responds
Davis' study is "interesting," Andrew Shao, PhD, tells WebMD. He's the vice president for scientific and regulatory affairs at the Council for Responsible Nutrition, a trade group for the supplements industry.
He points out that in Davis' study, the mice were on black cohosh throughout their adult lives, while women typically take black cohosh for several months.
Shao also notes conflicting data about black cohosh and breast cancer. He says some studies suggest that black cohosh "could even reduce the risk of breast cancer" and that "there are no human studies that suggest that it's a problem."
In their paper, Davis and colleagues acknowledge the mixed findings from other studies on black cohosh and breast cancer. But they note issues with some of those studies, such as a lack of a comparison group in a German study of breast cancer patients and a high dose of black cohosh extract used in a study done on cells.
Shao admits that there haven't been any randomized clinical trials to directly test black cohosh for its effects on breast cancer in people.
Shao's advice: "If one does have breast cancer and is looking to try to alleviate some of the symptoms associated with menopause, be in touch with your practitioner, your health care provider. Don't make these decisions in a vacuum."