Two drugs are equal in preventing breast cancer.

A commonly prescribed anti-osteoporosis drug works as well at preventing breast cancer in postmenopausal women as the sole drug currently prescribed for the task, a head-to-head trial shows.

Scientists designed the study to compare oral doses of the osteoporosis drug raloxifene (Evista) with tamoxifen (Nolvadex) taken for 5 years. Roughly half of the nearly 20,000 women received raloxifene; the others got tamoxifen. All the women were at high risk of breast cancer.

Researchers report that roughly the same number of women--167 and 163--developed breast cancer while taking raloxifene and tamoxifen, respectively.

However, 36 women getting tamoxifen developed uterine cancer, compared with only 23 taking raloxifene. Also, more women getting tamoxifen developed blood clots than did women taking raloxifene.

Researchers stopped the trial after 4 years of average follow-up when the study established the drugs' equal effect on breast cancer development. The National Cancer Institute, which funded the trial, released the results in mid-April.

Research has shown that the hormone estrogen binds to cells and stimulates their proliferation in about 70 percent of breast cancers. Both tamoxifen and raloxifene block the hormone from binding to breast cells.

"It's clear that raloxifene is the winner" in the new study, says Lawrence Wickerham, an oncologist at Allegheny General Hospital in Pittsburgh.--N.S.