Last Updated: 2003-07-14 12:45:27 -0400 (Reuters Health)

WASHINGTON (Reuters) - Black cohosh, a herb popular for relieving the hot flashes and some other unpleasant symptoms of menopause, may make cancer more likely to spread, U.S. and Canadian researchers said on Saturday.

The news is yet another blow to women looking for something to safely ease the symptoms of menopause, which range from hot flashes to a higher risk of heart disease and cancer.

Hormone replacement therapy has been found to worsen the risk of heart disease, stroke, some cancers and Alzheimer's, while some herbal supplements, such as red clover, have been found to do no good.

Many women had been taking black cohosh because it seemed to ease the hot flashes and, because it was herbal, many presumed it was safe. Not so, researcher Vicki Davis of the Mylan School of Pharmacy at Duquesne University in Pittsburgh told a meeting of the American Association for Cancer Research.

She said taking the supplement could endanger a woman who did not know she had cancer. "This stresses that we really need more research into herbal therapies and natural therapies."

Davis and colleagues in her lab and in Canada fed black cohosh to female mice bred so that they are prone to breast cancer. They gave them the daily equivalent of 40 mg of the supplement, the amount normally recommended for menopausal symptoms.

The mice were not any more likely to develop breast cancer in the first place. But those that did develop it were more likely to see a deadly spread of the cancer, Davis said.

Her team found that 27 percent of mice that ate black cohosh had the cancer spread to the lung, compared to 11 percent of the mice that did not eat the herb.

The findings do not prove that using black cohosh could endanger a woman with undiagnosed breast cancer, but Davis said it would be risky to take it. Women who have breast cancer -- or any other form of cancer -- may need to be especially wary, she said.

It is not clear how black cohosh may make cancer more likely to spread, Davis said.

"And one thing we don't know is whether it might accelerate a tumor that might be likely to metastasize or increase the numbers of tumors that would metastasize. But neither one is good," she said.