A study, which evaluated tamoxifen use in 1,103 breast cancer survivors who were initially diagnosed with estrogen receptor (ER)-positive breast cancer, and found that 369 patients developed a second breast cancer. Tamoxifen reduced ER-positive second breast cancers but boosted the risk of the less common but more aggressive ER-negative tumors. The study found that women who took the drug for at least five years had a 4.4-fold higher risk of ER-negative breast cancer however there was no link to ER-negative breast cancer risk if tamoxifen was used for less than 5 years. Researchers have emphasized that the study wasn"t designed to show a woman"s absolute risk of developing an ER-negative breast cancer and that it is not a controlled clinical trial that is considered the gold standard in medicine. They also said that the findings should not affect practice because the drug"s benefits were well established.
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