Hi @Milly21 , I have had lower back pain on and off whilst taking tamoxifen but mine felt more like an internal pressure and I attributed that to being forced into menopause. I do get sore knees (particularly left knee) and incredible muscle cramps. I have found that for my knees which are most sore at night in bed -I take panadol osteo and I have elevated my knees on a bolster cushion. It's amazing what a difference that has made to my sleeping. I think I would still go and get checked out though - peace of mind is invaluable. Big hug. Xx Cath