As primek says, hormonal treatment blocks oestrogen and that affects bone density. My oncologist thinks I'll make 5 years (on Femara) without any significant damage, but my bone density was remarkably good for my age when I started. I am nearly 4 years on and the reduction is noticeable in tests but not having any obvious impact otherwise - no aching, no arthritis. I am doing some moderate work with weights at the gym as this may help, as will any weight bearing exercise. I was aware of this likely effect when I started treatment, so no surprise and I still think it's worth doing but information before you start treatment should be the norm.