Why women taking tamoxifen suffer from the risk of developing other cancers of the reproductive tract in comparison to all other women ?

Hormone effect: In the breast tamoxifen blocks any estrogen activity but in the uterus it is just the opposite where tam stimulates the uterine lining just like estrogen. This can causes overgrowth of the lining (endometrial hyperplasia) which is a risk for cancer. The same risk is present with hormone replacement with estrogen only which is why progesterone needs to be given as well with HRT.