Genistein, one of the components of soy often touted for its possible beneficial effects, may in fact be harmful for some women with breast cancer, according to a new study.

In a study conducted in mice, researchers found that when taken as part of a daily diet, genistein can stop the ability of tamoxifen to halt breast cancer growth. The results were reported in the journal Cancer Research (Vol. 62: 2474-2477).

Of the 203,500 cases of breast cancer expected to be diagnosed this year, most will be estrogen dependent. These cancers grow partly in response to estrogen (the main female hormone). They are described as estrogen receptor positive, or ER positive, cancers.

Tamoxifen has long been an effective treatment for women with ER positive breast cancer. It works by blocking the estrogen receptors and preventing estrogen from attaching to them.

Tamoxifen is mostly used after surgery or radiation therapy in women who are past menopause. It reduces the risk that the cancer will return. Tamoxifen is also used by some women without breast cancer but who are at high risk of developing it, because it helps lower this risk.

But tamoxifen is not without side effects. Some women who take it feel menopause-like symptoms, including hot flashes and emotional changes. There are few useful treatments for these symptoms.

Doctors usually advise women with a history of breast cancer to avoid hormone replacement therapy (HRT) — which can lessen the symptoms — because it includes estrogen and could have an effect on any lingering breast cancer cells.

Some women turn to soy products or supplements to lessen their symptoms, even though good scientific proof that they work is lacking.

Soy contains substances called phytoestrogens, such as genistein, which can act as estrogens in the body. But these compounds might counteract the tamoxifen itself, making it less effective and possibly causing breast cancer cells to grow.

To test this theory, researchers from the University of Illinois at Urbana-Champaign inserted estrogen-dependent breast cancer cells into lab mice. When a group of these mice was then implanted with estrogen, the breast cancer tumors began to grow quickly.

In groups where both estrogen and tamoxifen implants were used, little tumor growth was seen.

But when mice with estrogen and tamoxifen implants were fed a diet high in genistein, breast cancer tumors again developed, showing that the protection of tamoxifen had been lost. The researchers noted that the levels of genistein used were "well within the range of reported human exposures."

The researchers were quick to say that further studies are needed to clarify the interaction between genistein, estrogen, and tamoxifen.

In particular, the dose range at which genistein affects tamoxifen should be studied, so that recommendations based on research can be made.

In the meantime, they said, women taking tamoxifen for breast cancer should use caution when it comes to genistein.

Women concerned about such issues should openly discuss any supplements they are taking with their doctors, experts advise.

Complementary and alternative forms of therapy are becoming more popular in the US. Some studies report that somewhere between 28% and 91% of women with breast cancer take some type of supplement not prescribed by their doctors.

Many do not let their doctors know what they are taking. This could lead to less effective therapies, or even harmful interactions.