A popular herbal remedy may make breast cancer chemotherapy more toxic, laboratory studies suggest.

The herb, black cohosh, has been used for centuries as a remedy for hot flashes and other symptoms of menopause. It was the main ingredient in the once-popular Lydia Pinkham's Vegetable Compound.

Women with breast cancer — or at high risk of breast cancer — should talk to their doctors about treatment for menopausal symptoms. Many women with breast cancer take anti-estrogen drugs that, as a side effect, cause menopausal symptoms like hot flashes. So the use of black cohosh is widespread among breast cancer patients, who may not tell their doctors they are using an herbal remedy.

Might this affect cancer chemotherapy? Sara Rockwell, PhD, Yale University professor of therapeutic radiology and pharmacology, took a look.

Rockwell's team grew breast cancer cells from mice in their laboratory. They tested three commercially available, liquid extracts of black cohosh. None of the extracts, by themselves, harmed the mouse cancer cells.

But when combined with breast cancer chemotherapy, the extracts had different effects. The black cohosh extracts increased the cell-killing effects of two drugs, doxorubicin and docetaxel. They decreased the cell-killing effects of another drug, cisplatin. They had no effect on cells treated with radiation.

Although very high doses of the black cohosh extracts were used in most of these experiments, significant effects were seen at concentrations equal to about 2.5 times the recommended dose. That's well within the dose range used by some women.

It's possible, of course, that these effects could enhance breast cancer chemotherapy. But it's also possible these effects could be life-threatening, Rockwell and colleagues suggest.

"Our studies caution that black cohosh should not be considered to be a harmless herb that is inconsequential to the health of cancer patients or to the outcome of conventional cancer therapy," they write. "Until the effects of black cohosh are better defined, the use of this and similar herbal preparations by breast cancer patients should be avoided."

The study appears in the April issue of Breast Cancer Research and Treatment.

By Daniel J. DeNoon, reviewed by Brunilda Nazario, MD

SOURCES: Rockwell, S. Breast Cancer Research and Treatment, April 2005; vol 90: pp 233-239. News release, Yale University. WebMD Medical News: "Dangers of Black Cohosh as HRT."