Hi ROSG, welcome to the site. When I was moved from Tamoxifen to Arimidex some months back there was some concern about the effects on my bone density as I have borderline osteopenia so there were a few things my oncologist did:
1) ordered a bone density scan that, when compared to a previous scan, showed my bone density as stable - if it had continued to decline she would have left me on Tamoxifen, which does offer some protection for bones,
2) put me on a bone thickening drug (Alendrobell) that I had to have once a week, and
3) continued advising me on the need for Vitamin D, exercise etc.
All was going OK until I had some unexplained bleeding which possibly indicated I wasn't entirely post-menopausal, so I'm back on the Tamoxifen until February as Arimidex is not effective with pre-menopausal women. If I'm going back on Arimidex in February my oncologist will closely monitor my bone density (yearly scans) and will put me back on the bone thickening drug.
Two of my tumours were strongly hormone positive (as per the pathology report), so either Tamoxifen or Arimidex (or similar) are an important part of my continuing treatment and although the side effects were initially pretty horrible (constant hot flushes, night sweats, joint pain etc) they have settled. As suggested, it would be good to chat to your oncologist to understand more about the role of hormone therapy in your treatment plan and why it is important for you. Best wishes, Jane xx