Women who take soy or herbal supplements, such as black cohosh, red clover and ginseng, should do so with care, says a Cornell University expert affiliated with the Program on Breast Cancer and Environmental Risk Factors (BCERF) at Cornell, the land-grant institution of New York state.
"Although there is no direct evidence that any herbal medicines can increase or decrease breast cancer risk, some herbs can have estrogen-like actions and thus raise concern about their long-term use," said Barbour Warren, a research associate with BCERF and the co-author of the fact sheet "Herbal Medicines and Breast Cancer Risk," which is available free on BCERF's Web site.
"Some of these substances may prove to have beneficial effects -- and, in fact, may become part of conventional medicine in the future -- but for now, the lack of any regulatory oversight has set up a marketplace for herbal supplements, which is such a mess that women should have lots of reservations in taking these supplements," Warren said.
He points to not only a lack of well-designed clinical trials but also to a lack of any regulations regarding effectiveness or quality control in formulating these supplements and says that women with a high risk of breast cancer should be particularly cautious.
According to Warren, 23 percent of middle-aged women in the United States use herbal medicines, often to treat premenstrual or menopausal symptoms and premenstrual syndrome, to aid in breastfeeding or to decrease breast cancer risk.
Warren reviewed the literature on herbal supplements and breast cancer risk for his report, which was reviewed by two international experts in the field as well as the five scientists on the BCERF staff.
"Just because herbal medicines are 'natural' products does not mean that they are safer than conventional medicines," he said. "The ingredients in herbal medicines can also have adverse effects and lead to health problems. Yet, there is no focused safety testing of herbal medicines or even reporting system for adverse effects, so when people do have adverse effects, there's no mechanism to pool that data."
Warren said that researchers used to think that estrogen-like compounds from plants, called phytoestrogens, could possibly block the effect of estrogen in the body and perhaps reduce breast cancer risk. However, recent clinical studies have shown that women on diets high in soy phytoestrogens experience greater cell multiplication in the breast. There is concern over this effect since it could be a preliminary step in cancer formation by leading to the outgrowth of latent cancer cells.
In looking at studies on the use of supplements for menopause, Warren reports that the seven clinical trials using black cohosh were largely flawed but might suggest some effectiveness. However, little evidence exists for beneficial effects of red clover, and no benefit was reported for ginseng, evening primrose oil or dong quai. In the 16 studies that looked at soy supplements, twice as many studies reported no effect as those that reported a beneficial effect, and although kava kava has been reported to have some benefit, reports of serious side effects have led to it being banned for sale in the United States and elsewhere.
For premenstrual syndrome, several studies found some benefit in about half the women studied for evening primrose oil, but two of the best designed studies found no effect; ginkgo biloba extract was found to have no effect, while a beneficial response from chaste tree berry extracts was found in about half the women studied. For menstrual pain, two clinical trials have examined the Japanese herbal preparations toki-shakuyakusan and toki-shakuyakusan with shakuyaku-kanzoto; neither found a beneficial effect.
"Some herbal medicines show promise for the treatment of problems with menstruation and menopause and other conditions that affect women," said Warren. "It is unfortunate that their quality and safety are not better controlled. Such an environment has set up a situation where there are potentially many accidents waiting to happen."
However, he said, if women choose to use herbal medicines, they should do so cautiously because the quality of these medicines varies, and the safety, especially with long-term use, is uncertain. He said that women should choose only high-quality products from reputable sources and to be sure to inform their health-care providers about their herbal medication use, because many herbal supplements can interact with other treatments.