The study, published in Cancer Research, compared breast-cancer patients who received tamoxifen for at least one year with those who did not take the drug. The researchers, from the Fred Hutchinson Cancer Research Center in Seattle, found that the drug was associated with a 60 percent reduction in the risk of developing a second estrogen receptor-positive breast cancer. But tamoxifen appeared to increase the risk of developing an estrogen receptor-negative breast cancer by 440 percent. However this finding should not discourage women from taking tamoxifen to prevent a second cancer, said the lead author of the study, Dr. Christopher Li. 'It is clear that estrogen-blocking drugs like tamoxifen have important clinical benefits and have led to major improvements in breast cancer survival rates,' Li said in a news release. 'However, these therapies have risks, and an increased risk of ER-negative second cancer may be one of them. Still, the benefits of the therapy are well established and doctors should continue to recommend hormonal therapy for breast cancer patients who can benefit from it.' 