Potential interaction with dietary supplement genistein and breast cancer drugs.




Cancer Research

Related Monographs

Consumer Data: Soy Isoflavones Breast Cancer
Professional Data: Soy Isoflavones Breast Cancer


The incidence of breast cancer has been increasing steadily for decades. In 1972 when President Nixon declared our national war on cancer, a woman's lifetime risk of developing breast cancer was 1 in 20. Today breast cancer rates have escalated to the point where women's lifetime risk of developing breast cancer is 1 in 8. In the year 2000, the American Cancer Society estimates that nearly 182,800 women will be diagnosed with breast cancer and approximately 40,800 women will die from it. This means that every three minutes a woman in the United States is diagnosed with breast cancer and every twelve minutes, a woman dies from this dreaded disease. Breast cancer has become the second largest cause of cancer death in women, after skin cancer, and the leading cause of death for women between the ages of 35 and 54. Consequently, some physicians and research scientists are now referring to this as the breast cancer epidemic.

Conventional treatment of breast cancer may involve the use of anti-estrogen medications. One of these drugs is tamoxifen. This medication appears to act by binding to estrogen receptor sites on cells. It has been used for over 20 years to treat estrogen receptor positive breast cancer and is probably the most commonly prescribed anticancer medication in the world.

With the increasing use of dietary supplements, there is an increasing risk of drug and supplement interactions. Soy isoflavones are a popular supplement used by postmenopausal women as well as women with breast cancer. Researchers recently investigated the potential interaction between the isolfavone genistein and the anti-cancer medication tamoxifen. This animal study analyzed the growth of breast cancer cells in mice with the administration of genistein and tamoxifen. The mice were divided into 6 treatment groups and the researchers found that genistein negated the inhibitory effects of tamoxifen in tumor growth. The authors concluded by warning that women who take isoflavones while receiving tamoxifen anti-estrogen breast cancer therapy should be cautious.1


1. Ju YH, et al. Dietary Genistein Negates the Inhibitory Effect of Tamoxifen on Growth of Estrogen-dependent Human Breast Cancer (MCF-7) Cells Implanted in Athymic Mice. Cancer Research. May 2002;62:2474-2477.