I switched because after less than a year on Anastrozole I was having debilitating knee pain which made stairs and even getting up and down from a seating position nearly impossible. I had been on Letrozole prior to surgery with few side effects, but new oncologist switched me to Anastrazole. Now I take Letrozole at half dosage (though I'm currently on a 30 day break to see if other issues improve.) After a few weeks off Anastrozole there was definite improvement and then I added collagen supplements. Within a few monyhs my knees felt almost brand new.