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NEW YORK (Reuters Health) - Only a small number of U.S. women at high risk of breast cancer have chosen to use the drug tamoxifen to lower that risk, according to a study published Monday.In 2005, government researchers found, about 51,500 U.S. women were using tamoxifen in order to lower their risk of breast cancer.For comparison, findings from a 2000 government health survey indicated that 10 million U.S. women were medically eligible for the drug, and that for 2.4 million of those women, the risk-reduction benefits of tamoxifen would likely outweigh the risks of side effects.Exactly why so few women are using tamoxifen to curb breast cancer risk cannot be gleaned from the data, said Dr. Andrew N. Freedman of the U.S. National Cancer Institute, one of the researchers on the study.On one hand, he told Reuters Health, the findings could indicate that many doctors are not discussing tamoxifen with patients who are candidates for it, or that they are reluctant to prescribe it.Conversely, he said, those discussions may in fact be happening, and many women are opting not to take the drug because of its potential risks."This study really just describes how many women are using tamoxifen" to prevent breast cancer, Freedman said. It does not answer the question of whether those numbers can be considered "too low" or "correct," he noted.Freedman and his colleagues report the findings in the journal Cancer Epidemiology, Biomarkers & Prevention.Tamoxifen, which works by blocking estrogen activity in breast tissue, has long been used to help treat breast cancer. In 1998, the U.S. Food and Drug Administration (FDA) approved the drug for the prevention of breast cancer in women age 35 and older at increased risk of the disease -- defined as having at least a 1.67 percent chance of developing breast cancer in the next five years.Doctors estimate that risk by looking at a combination of factors, including age, whether a woman has ever had a first-degree relative diagnosed with breast cancer, and whether she has ever needed a breast biopsy to evaluate tissue abnormalities.The FDA approval came after a U.S. clinical trial of more than 13,000 high-risk women found that those randomly assigned to take tamoxifen for five years were half as likely to develop invasive breast cancer as those given a placebo.However, the trial also revealed a number of risks, including heightened rates of endometrial cancer, blood clots and cataracts among tamoxifen users. For example, over seven years of follow-up, 53 women in the tamoxifen group developed endometrial cancer, versus 17 women in the placebo group. And 28 tamoxifen users developed a blood clot in the lungs, compared with 13 cases in the placebo group.Tamoxifen can also cause menopausal-like symptoms like hot flashes and vaginal dryness.In deciding whether to use the drug, Freedman said, high-risk women also have to consider factors like their overall health, any other medications they are taking and, more personally, their own preferences."It's a personal decision," Freedman said, "and it requires an in-depth discussion with your doctor."He added that women should also be aware that there is another drug option -- raloxifene (Evista), an osteoporosis drug that was approved in 2007 for breast cancer prevention in postmenopausal women. Like tamoxifen, it blocks estrogen activity in breast tissue, and studies so far indicate that the two drugs are similarly effective at reducing breast cancer risk.Raloxifene is known to raise the risk of blood clots. But in a 2006 U.S. clinical trial comparing the drug with tamoxifen for breast cancer prevention, raloxifene users had fewer blood clots, endometrial cancers and cataracts.The current findings are based on data from the National Health Interview Survey, a periodic government survey of a nationally representative sample of Americans.In 2000, Freedman and his colleagues estimate, 120,737 U.S. women between the ages of 40 and 79 were using tamoxifen to cut their risk of breast cancer. Five years later, that figure was 51,575. As a percentage of the population studied, the difference between the two years was not significant in statistical terms, according to the researchers.It would be worthwhile, Freedman said, for future studies to delve into the reasons for the relatively low rate of tamoxifen use.