By Neil Sherman
HealthSCOUT Reporter

THURSDAY, Oct. 7 WESTPORT, Conn. (HealthScout) — If you're using tamoxifen to fight breast cancer, you've probably been warned of your risk for uterine cancer. New research now shows that women using estrogen-replacement therapy, as well as obese women, are at highest risk for tamoxifen's side effect.

Experts caution, however, this is no reason to stop using tamoxifen.

"We don't want people to throw out tamoxifen, like the baby with the bath water," says Leslie Bernstein, professor of preventive medicine at the University of Southern California in Los Angeles, and lead author of the study published in the Journal of the National Cancer Institute. "It's an incredible drug -- it may prevent the development of breast cancer, it reduces the risk of breast cancer recurrence and it extends survival rates."

Tamoxifen's risks are well known, says Dr. Lawrence Wickerham, associate chairman of the National Surgical Adjuvant Breast and Bowel Project at the University of Pittsburgh. "But I agree with the study's findings that there is no reason to either stop using estrogen or to discontinue the use of tamoxifen."

"The study provides a refinement," Wickerham says. "It helps us gain an understanding of those patients who may have a greater-than-average chance of getting endometrial cancer [cancer of the lining of the uterus], and it should provide doctors with a heightened awareness and create a new focus on endometrial care."

Tamoxifen is "one of those designer drugs -- a hormone as well as an anti-hormone," Bernstein explains. "The drug binds to estrogen receptors and blocks the effect of estrogen, which then reduces the proliferation of cancer cells in the breast."

Unfortunately, tamoxifen also has an estrogen-like effect on the lining of the uterus "in a tiny proportion of women taking the drug," Bernstein says.

The drug increases risk for endometrial cancer, the most common gynecologic cancer in women. It will be diagnosed in 37,400 women this year and will cause 6,400 deaths.

In a controlled study, Bernstein compared 324 breast-cancer patients who had developed endometrial cancer with 700 women who had had breast cancer but had not developed uterine cancer. The study systematically explored subjects' medical records, matching the first diagnosis of breast cancer with age, race and controls.

The results: tamoxifen increased the risk for endometrial cancer by about 50 percent in women who had had estrogen-replacement therapy or in women who are heavy.

The longer the tamoxifen therapy, the greater the risk. Women taking tamoxifen for more than five years were four times more likely to develop endometrial cancer than women who had not taken the hormone.

Women who are overweight, Bernstein says, were shown to have "the greatest risk -- it was incredibly high."

The study should help doctors identify breast-cancer patients with a higher risk for endometrial cancer, Bernstein says. "There's a group doctors will have to screen thoroughly, and there is a group that they may not have to screen."