A newer drug prevents breast cancer in older, high-risk women just as well as today's standby tamoxifen — but with fewer side effects, the National Cancer Institute announced Monday.

Called raloxifene, the newer drug already is sold to treat bone-thinning osteoporosis.

But the striking new results, from a government study of nearly 20,000 women, suggest that raloxifene may supplant its older cousin as the first choice for breast cancer prevention in postmenopausal women at high risk of developing the disease.

"Now women have a choice," Dr. Leslie Ford, NCI's cancer prevention chief, stressed in an interview Monday. "It's good news, because we're giving you a choice with fewer side effects."

Manufacturer Eli Lilly & Co., which sells raloxifene under the brand name Evista, plans to seek Food and Drug Administration approval for the new use.

Until now, tamoxifen has been the only drug approved to reduce the chances of breast cancer striking high-risk women. It's a multiple-use drug: It acts like the estrogen hormone in some tissues, but like an anti-estrogen in others.

Estrogen can fuel certain breast cancers, making tamoxifen a longtime top choice both to prevent the disease's return in women with estrogen-sensitive tumors and to reduce the odds of it striking high-risk women in the first place.

But tamoxifen causes some rare but serious side effects: It acts like an estrogen in the uterus and bloodstream, thus increasing users' risk of getting uterine cancer or a life-threatening blood clot.

Raloxifene is a close chemical relative, in the same drug family known as "selective estrogen response modulators," and earlier research suggested that it might help breast cancer, too. So the NCI launched the $88 million study to compare the two.

Taking either tamoxifen or raloxifene every day cut in half women's chances of developing invasive breast cancer, NCI announced Monday.

Raloxifene causes the same side effects, but not as many: Raloxifene users had 36 percent fewer uterine cancers and 29 percent fewer blood clots, the initial results show.

Some 2 million U.S. women every year are thought to be candidates for tamoxifen risk-reduction therapy, but many have avoided it for fear of those side effects, said Dr. Kathy Albain of Loyola University, a researcher in the new study.

"Here we have something that's a little less scary," she said of the raloxifene findings. "It might tip the scales for a lot of women."

The new study means no change for premenopausal women — there's no data showing whether raloxifene is safe for them, Albain stressed.

Nor does it mean that tamoxifen users should necessarily switch, she said. Women currently are prescribed tamoxifen for five years, and its breast cancer prevention benefit continues even after they stop taking the drug — as raloxifene's seems to. So a woman already in, say, year 4 of her tamoxifen course with no sign of side effects probably has little to gain by switching, she explained.

Still, that's a question researchers were girding for as they spent Monday notifying study participants of the results.

Among postmenopausal women, who's at high risk? NCI defines it as someone with at least a 2 percent chance of getting breast cancer within five years — because of advanced age, a close relative with the disease, never having a child or having one late in life, or other well-known risk factors that women can calculate on a government Web site: http://cancer.gov/bcrisktool.

Most of the study participants actually had a far higher risk, Ford said: For every 1,000 of them, doctors expected 40 to develop breast cancer within five years if they did nothing. Taking one of the drugs cut that number to 20 instead.