I am on Arimidex, which my oncologist says is very effective. Tamoxifen has certain side effects that were a concern for me. The first one was that I had had a pulmonary embolism right after my cancer surgery. If you have a higher risk for that, Arimidex is better. Also, Tamoxifen is rougher on people with heart-related problems. How old is your mom? Is her general health good? Another factor, of course, is what type of tumor she had, and if she is postmenopausal. Arimidex will only be prescribed if she has gone through menopause. Hope this is helpful.