I also had herceptin (infusion) for a year and have been on letrozole for six years. I too was warned about arthritis type aches and pains. I have had none, but others have. No idea what makes the difference. There are two other notable side effects - reduction in bone density and vaginal dryness. My bone density was excellent before starting and is still
on the right side of the ledger but is being closely monitored and may yet stop me going to ten years.
Vaginal dryness is a pain in the vagina! I have just begun using oestrogen cream as I was very wary of doing something that seems completely against current treatment. It helps, but I am still cautious.
Not everyone gets the dryness, so my usual advice is a) follow your oncologist's advice if you trust him/her b) if you try something and are really unhappy with it, you can always stop! Best wishes.