The variation is possible. It’s also true that letrozole can have joint pain as a side effect. I get quite a bit short with chemists giving generic brands if not asked specifically to do so, they have no way of knowing your oncologist’s reasoning for prescribing a specific medication. It’s often done because the other brand is cheaper but even so....It’s certainly worth trying Femara instead, but the difference is in the binding materials (both have the same therapeutic ingredient), and it’s hard to know what you may be reacting to. I have been on Femara for 7 years and have no joint pain (do have other side effects) but I had no arthritis or joint pain prior either. Give Femara a shot but if pain persists, you may need to talk again with your oncologist about options. Best wishes.