What Is the Connection between Tamoxifen and Endometrial Cancer?

Tamoxifen is a drug that is commonly used to treat breast cancer. Its effects on estrogen in the human body can create an increased risk of developing two forms of uterine cancer. Endometrial cancer, affecting the uterine lining, and a cancer of the uterine wall, called uterine sarcoma, both become about 50 percent more likely following therapy with this drug. The link between tamoxifen and endometrial cancer is one of the reasons that this uterine syndrome is the most common cancer to affect the female reproductive system.

This medication interferes with the action of the hormone estrogen in the breast. Certain types of breast cancer cells grow best in the presence of estrogen, so tamoxifen essentially prevents these cancer cells from dividing and growing. In other types of tissue, including uterine tissue, this medication behaves like estrogen instead of opposing it. Uterine cancers are associated with elevated estrogen levels, so the connection between tamoxifen and endometrial cancer may be related to the drug's hormone-like action in the uterus.

Tamoxifen and endometrial cancer development seems to be related to the length of time the medication was used. Breast cancer treatment for tumors detected early may sometimes involve up to five years of chemotherapy. This length of time seems to prevent a later recurrence of breast cancer. Studies by the University of Southern California found that taking this drug for five years or more tended to drastically increase the risk of endometrial cancer, however.

The normal rate of endometrial cancer among women is estimated at roughly 1 in 1,000, according to the American National Cancer Institute. After five years or more of chemotherapy, this rate increases to about 2 in 1,000. Statistically then, there is a clear correlation between tamoxifen and endometrial cancer, even if the exact mechanism by which the cancer forms is not yet known.

Women using this drug for chemotherapy should be aware of certain risk factors that can strengthen the link between tamoxifen and endometrial cancer. Estrogen replacement therapy was found by researchers as the University of Southern California to be the single factor that increased uterine cancer risk the most, in conjunction with taking this drug. Obese women also tended to have a much greater risk of developing this type of cancer. Taking birth control pills, which can reduce estrogen levels in the body, was found to offset some of the cancer risk posed by tamoxifen.