Although estrogen is used as a treatment for many women, it is also attributed to be a carcinogen. The National Toxicology Program voted that steroidal estrogen should be listed as a cancer-causing substance because of its association with endometrial and breast cancer. Steroidal estrogen is used in post-menopausal treatments and birth control pills. 

Doctors have known about the cancer link and usually prescribe post-menopausal estrogen pills along with another hormone called progestin. This combination lowers the risk of endometrial cancer. 

The National Institutes of Health updates the federal list of proven and suspected cancer-causing substances every two years. 