There is no evidence as to estrogen CAUSING breast cancer and some findings challenge seriously the notion that estrogen can increase the risk as estrogen alone in two large randomized trials significantly reduced the risk in women and also in women close to menopause taking E and a progestin. Pregnancy, long-term, has been associated with a reduced frequency of breast cancer despite very high levels of E. Also, risk of breast cancer is highest in older women who have declining levels of E. Women taking E after having breast cancer did not show an increase in recurrence relative to those taking no E. E has also been used to treat breast cancer, with some success, sometimes in very high doses. Several conflicting studies.
In a recent study, it was concluded that the risk of breast cancer did not increase in transsexual women despite the use of estrogen, in sometimes very high doses in the past. The risk was deemed similar to cismen not taking HRT and lower relative to ciswomen.
