Probably in the short term, but results of ongoing trials are needed to determine longer-term safety
Oestrogens play an important role in the development of breast cancer. This is most evident in postmenopausal women: circulating levels of endogenous oestradiol are higher in those who develop breast cancer,1 while use of hormone replacement therapy (HRT) increases breast cancer risk.2 Recent results from the Women's Health Initiative randomised trial showed a 26% excess rate of breast cancer development in women who took combined continuous equine oestrogens and medroxyprogesterone acetate for a mean of 5.2 years compared with placebo.3 This finding is consistent with results of earlier epidemiological studies that suggest breast cancer incidence is increased more by combined preparations than by oestrogen alone.2 Further evidence that oestrogen is important in breast cancer development comes from a study of over 9300 postmenopausal women with early breast cancer.4 This found that anastrozole (an aromatase inhibitor that dramatically reduces oestrogen production) significantly reduced the rate of new contralateral breast cancers compared with tamoxifen (hazard ratio, 0.42; 95% CI, 0.22–0.79; P = 0.005).4
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