The three-year study of 7,705 women found a 76 percent lower risk of breast cancer among those taking raloxifene compared with those given a placebo. Raloxifene is marketed under the name Evista by Eli Lilly and Co., which paid for the study.
"It's not a surprise that it reduces the risk of breast cancer, but the magnitude of this effect is really remarkable to me," says Dr. Steven R. Cummings, a professor of medicine and epidemiology at the University of California at San Francisco who led the study. "Seventy-six percent reductions are not something we find in medicine very often."
Raloxifene is part of a new generation of drugs that scientists hope will mimic the good effects of estrogen -- stronger bones and a lower risk of heart disease -- while inhibiting the possible harmful effects, which may include promoting breast and uterine cancer.
The Food and Drug Administration approved raloxifene in 1997 for preventing osteoporosis. It has not been shown to lower the risk of heart disease.
Although researchers said Tuesday they were not yet prepared to recommend ramoxifene for breast cancer prevention, the drug may one day become an alternative to tamoxifen, an anti-cancer drug not suitable for all women, reports CBS News Correspondent Bobbi Harley. However, some doctors are prescribing raloxifene to women who have had breast cancer or precancerous cells removed, but who are afraid to take tamoxifen because of the risks of uterine cancer.
Many women are afraid to use estrogen because of conflicting evidence on whether it promotes breast cancer. But a study released last week on 37,000 women suggested that hormones do not increase the risk of breast cancer, except for some uncommon and highly curable forms of the disease.
Carmen Larossa has battled breast cancer for three years now, and it's something she hopes her daughter doesn't have to endure in the future.
"She's at high risk," Larossa says.
For those who participated in the trial of the drug, the new study is welcome information, reports CBS News Health Correspondent Dr. Emily Senay.
For Juanita McGuire of San Rafael, Calif., participating was important because breast cancer runs in her family. Two of her aunts died from the disease.
McGuire does not know whether she received the placebo or the real drug, and may not know for another year, since the trial is continuing to study further long-term effects.
"When you know there's cancer in the family or has been cancer in the family, if you can do anything that might prevent it happening, you should do it," McGuire says.
But there's new hope for these women. It started last year with tamoxifen, a drug long used to treat breast cancer that scientists disovered could also prevent it.
Unlike tamoxifen, which can cause blood clots and uterine cancer, the side effects of ramoxifene appear to be minimal. However, the drug isn't risk-free -- it increases the chances of serious blood clots.
In Cummings' study, 5,129 post-menopausal women younger than 81 received raloxifene daily, while 2,576 got a dummy pill. Thirteen cases of breast cancer were diagnosed among the women on raloxifene; 27 cases were found among those taking the placebo.
Women taking raloxifene had a 90 percent lower risk of a type of cancer called estrogen-receptor positive breast cancer.
However, raloxifene had almost no effect on estrogen-receptor negative breast cancer, one of the hardest forms of the disease to treat. It is most commonly developed by younger women and those with a genetic predisposition to the disease.
Dr. Janet Wolter of Rush-Presbyterian-St. Luke's Medical Center in Chicago, said she is encouraged by the findings and hopes more research will determine whether the breast cancer drug tamoxifen or raloxifene is more effective and produces fewer side effects.
An accompanying editorial called the findings encouraging but cautioned that raloxifene cannot yet be considered suitable for most women.
"Its contributions to knowledge intensify the anticipation of finding something even better on this new frontier," said Drs. Adele L. Franks of the Prudential Center for Health Care Research in Atlanta and Karen K. Steinberg of the Centers for Disease Control and Prevention.
This fall, doctors may learn more when thousands of women take part in a nationwide study in which the two therapies will be tested against each other and against the disease.