Hello @jixs stiffness and muscle pain is a known side effect of arimidex (Anastrazole). I am sorry that you are struggling with this.
I did take anastrazole for years, and I found that a walk once a day took all that stiffness away. I have read this same comment from others in this support group.
I honestly think letrozole has a worse set of side effects. With the nausea, and the dizziness or just feeling off balance. This is just one case but I felt like arimidex was at least a side effect I could do something about.
Have you tried anything to make the stiffness better? What helped, what didn’t? Have you talked to your doctor about it?