"I don't see a clear winner," but 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 published long-term results from the federally funded study on Monday. 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 Indianapolis-based Eli Lilly & Co., more selectively blocks estrogen. It is only for use 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 among women on Evista versus 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, the Texas cancer specialist, 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. For example, women might choose tamoxifen if they are at very high risk of breast cancer and have had hysterectomies so that uterine cancer is not a concern.
Marty Smith, 55, an insurance agent in Grandville, Mich., has used both drugs. Her sister and mother had breast cancer and a grandfather had male breast cancer. She switched to Evista after two years on tamoxifen because of worries about side effects.
"I thought, if there's something else that's going to give me equal and possibly better breast cancer prevention with less risk, then I was going to get on it," she said.