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What is the best drug for breast cancer?

Which drug (Letrozole or Tamoxifen) should be taken for breast cancer? Are there negative implications of changing drugs? Is it possible to determine for sure whether a woman is post-menopausal?
My mum was asked to take Letrozole last year as the doctor said that she is post-menopausal. However, when she attended her follow-up appointment recently, it was another doctor who saw her case, who told her that she should take Tamoxifen instead as she observes high levels of female hormones and thinks she has not menopaused. During this recent follow-up, there were no additional tests done (the doctor was looking at the same report?).

Firstly, are there implications of changing the long-term medication after a year of taking Letrozole? Also, is there no way to confirm if my mother is post-menopausal? Our main worries - Why was there a change in view, also, what if another doctor asks her to change the medication again in her following check-ups?  
Dr. Tan Y S .
4 Streak Score Doctors are able to put a streak together by performing an activity everyday
Dear @curiousdaughter I am sorry to hear about your mum's condition.
For a more thorough answer, please accompany your mother to the next appointment with the medical oncologist (not the surgeon) and ask the doctors directly. (Disclaimer, I'm a surgeon!)

Briefly, Tamoxifen and Letrozole are both suitable medications for patients with ER+ breast cancer. Only Tamoxifen may be used in a woman who still has menses. Once a lady has stopped having any menses, the oncologist may continue the patient on Tamoxifen, or may recommend a switch to letrozole. 
There are other consideration to side effects of both medications, such as abnormal bleeding from the womb, clots in the deep veins of the legs and bone loss. 
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