Older women at higher risk for breast cancer now have two good drug options for preventing the disease, but they will have to weigh trade-offs, a major study shows.

Tamoxifen, the longtime gold standard, is more effective and longer lasting, results show. But a newer drug, raloxifene, sold as Evista, is safer.

"I don't see a clear winner," rather two good choices with different risks and benefits, said Dr. Scott Lippman, a cancer specialist at the University of Texas M.D. Anderson Cancer Center. He is editor of Cancer Prevention Research, a journal that recently published long-term results from the federally funded study. They also were being presented at an American Association for Cancer Research meeting in Washington.

Tamoxifen is widely used to treat cancer once it's diagnosed, and Evista is used to treat osteoporosis. But the drugs have not found wide acceptance so far as cancer preventives. Doctors hope the findings will spur more high-risk women to consider taking one of the drugs.

They're not recommended for women at average risk of breast cancer. But for the millions who are at higher risk because of gene mutations, family history or other factors, they can make a dramatic difference.

Tamoxifen cut the chances of developing the most serious forms of breast cancer in half, the research shows, but with a higher risk of uterine cancer. Evista cut the cancer risk by 38 percent, with fewer uterine problems and other serious side effects.

"We've now documented that it's far less toxic" than tamoxifen, said study leader Dr. D. Lawrence Wickerham. He is a cancer specialist at Allegheny General Hospital in Pittsburgh who has consulted for makers of both drugs.

Tamoxifen has long been used to treat and prevent breast cancer. It blunts estrogen, which fuels the growth of most tumors that occur after menopause.

Evista, sold by Eli Lilly & Co., more selectively blocks estrogen. It is for use only after menopause; its safety and effectiveness before then are unknown.

Generic tamoxifen costs about 30 cents a day, versus up to $3 for Evista. Both can cause hot flashes.

The study, called STAR, compared them in nearly 20,000 postmenopausal women at higher risk of breast cancer. They took one drug or the other for about five years and then stopped (longer use is not known to be safe or good). After about seven years of follow-up, there were 310 cases of invasive breast cancer in women on Evista vs. 247 in those on tamoxifen. That works out to a 24 percent higher cancer rate for Evista users.

Uterine cancer developed in 65 tamoxifen users but in only 37 women on Evista. Twice as many women on tamoxifen had abnormal uterine growths that led to hysterectomies. Blood clots and cataracts also were less common with Evista.

Evista clearly is the safer drug, said V. Craig Jordan of Georgetown University, the scientist who led development of tamoxifen. However, Evista's breast cancer prevention benefits wane over time much more than tamoxifen's do.

Lippman agreed.

"It may be that with raloxifene, you need to continue to take it," he said. And even counting the additional uterine cancers that occurred with tamoxifen, its users still had 35 fewer invasive cancers overall than women on Evista.

It sets up a choice, he said.

STAR study: www.cancer.gov/star. Cancer risk calculator: http://cancer.gov/bcrisk tool