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WASHINGTON (Reuters) - Longer-term results from a head-to-head trial of two drugs that prevent breast cancer shows that tamoxifen works better than rival Eli Lilly and Co's Evista, but with a greater risk of some other cancers and blood clots.After nearly seven years of follow-up, researchers found that women who took tamoxifen for five years were less likely to develop breast cancer than those who took Evista, known generically as raloxifene.But they told a meeting of the American Association for Cancer Research that both drugs are useful and that women should be able to choose. Only tamoxifen is approved for women who have not yet gone through menopause, and many may not want to risk its side effects.Tamoxifen was originally sold by AstraZeneca Plc under the brand name Nolvadex and is now available generically. Both drugs are in a class known as selective estrogen receptor modulators.The researchers on the so-called STAR trial calculate that tamoxifen lowers the risk of breast cancer in high-risk women by 50 percent, compared with 38 percent for Evista.Overall, both drugs also saved lives."There is no statistically significant mortality difference between the two treatment groups," meaning any differences might have been due to chance, the team, called the National Surgical Adjuvant Breast and Bowel Project, wrote in the journal Cancer Prevention Research.After 81 months of follow-up, 236 women who took tamoxifen had died of any cause, compared with 202 who took raloxifene.When the researchers first reported their findings from the STAR trial of more than 19,000 women in 2006, there was little difference between the two drugs. Now the differences are clearer, perhaps because raloxifene is less potent than tamoxifen, said the researchers, who were led by Dr. Victor Vogel of the University of Pittsburgh."The updated results reported here demonstrate that after a median follow-up of 81 months, which represents 60 months of treatment plus an additional 21 months of follow-up, raloxifene no longer appears to be as effective as tamoxifen in preventing primary invasive breast cancer," the researchers concluded.SIDE EFFECTSTamoxifen's side effects are clearer after more years of study, however. It prevents breast cancer for as long as 15 years. But it raises the risk of rarer diseases such as uterine cancer, while Evista does not.Tamoxifen also raises the risk of blood clots in the leg - called deep-vein thrombosis - which can travel to the lungs to cause a pulmonary embolism."The superiority of tamoxifen over raloxifene in reducing breast cancer risk comes with a cost: significantly more endometrial cancers, hysterectomies for benign disease, thromboembolic events, and cataracts," the researchers wrote."These toxicities may be acceptable for the treatment of breast cancer but have proved to be a barrier to the use of tamoxifen for preventing primary breast cancers."Women therefore may be more likely to take raloxifene."Our results demonstrate that raloxifene (compared with tamoxifen) retains substantial benefit in reducing the risk of invasive breast cancer and has fewer life-threatening side effects, including significantly fewer endometrial cancers," the researchers wrote.Women are only eligible to take either drug if they have a high risk of breast cancer - for instance, if they have had cancer in one breast and want to prevent it from coming back in the other, if they have a genetic risk of breast cancer, or if they have had a benign tumor called ductal carcinoma in situ.