Drug prevents some cancer, poses risks.

For "the first time in history," evidence indicates "that breast cancer can not only be treated but also prevented," says Bernard Fisher of Allegheny University of the Health Sciences in Pittsburgh.

Fisher is referring to the finding that women at especially high risk of breast cancer appear to halve their chances of developing the malignancy by taking tamoxifen, a synthetic hormone. These results, reported Monday, come from a federally funded trial directed by Fisher.

For 25 years, tamoxifen has been used to limit breast cancer recurrence in women treated for the disease. In 1992, the National Cancer Institute (NCI) in Bethesda, Md., launched a trial to see whether the drug could also prevent the scourge in women who had never had the disease. Half of some 13,000 volunteers received the drug; half got an inactive substance, or placebo.

While 154 of the women taking the placebo developed invasive breast cancer during the trial, only 85 women who were taking tamoxifen did.

Hailing the trial as a "historic" success, NCI Director Richard D. Klausner also noted that the drug poses serious risks. Among the women taking tamoxifen, a known carcinogen (SN: 3/2/96, p. 132), 33 developed uterine cancer, compared to 14 in the placebo group. The 17 pulmonary blood clots among tamoxifen users were almost triple the number in the placebo group and included two deaths. Clearly, Klausner said, "this drug is far from ideal," and the need to evaluate its benefits and risks individually for each patient "cannot be emphasized enough."

The increased risks showed up only in women over 50, notes NCI's Leslie Ford. Moreover, "none of the risks were unanticipated, and none were higher than expected" (SN; 5/9/92, p. 309).

Because two earlier studies suggested that taking tamoxifen for many years increases cancer rates and mortality (SN: 12/9/95, p. 391), NCI recommends that breast cancer survivors take it for no more than 5 years. Klausner said no one knows whether cancerfree women will need to adhere to the same limit.

"I'm glad they found a very substantial benefit in [breast] cancer prevention, because the drug's adverse effects in healthy women are very troubling," says breast cancer epidemiologist Trudy Bush of the University of Maryland in Baltimore.

Adds Cynthia Pearson of the National Women's Health Network in Washington, D.C., it's too early to know whether the tamoxifen group's lower cancer rate represents "a true prevention of disease versus just a delay."