Hi @Kmakm, I developed very bad lower back pain about 2 months after starting Letrozole & put up with it for some time worried it was something sinister. Eventually it went, that was 12 months ago & then recently developed it again although not as bad. I think cold weather aggravates it. Hot baths with magnesium help & heat packs. i wouldn't worry too much unless the pain is ongoing & bad enough for painkillers. My oncologist said if pain wakes you in the night or doesn't go away with rest there may be something to worry about & if you do enough research you will find other people have back pain too with Letrozole. Hope this helps.