The risk of endometrial cancer increases with the duration of tamoxifen treatment for breast cancer, continues even after treatment ceases, and is comparable in pre- and postmenopausal women, according to a case–control study conducted in the UK.  

Researchers compared 813 women who developed endometrial cancer after being diagnosed with breast cancer with 1,067 control women who developed breast cancer but no subsequent endometrial cancer.  

Overall, women who received tamoxifen were almost two and a half times more likely to develop endometrial cancer as women who did not receive treatment (OR=2.4; 95% CI: 1.8–3.0). The risk increased with duration of treatment, so that those who received treatment for 5 or more years were 3.6 times as likely to develop endometrial cancer as women who never received treatment with tamoxifen (OR=3.6; 95% CI: 2.6–4.8). And the risk did not diminish in follow-up to at least 5 years after the last treatment. This threat was not linked with the daily dose of tamoxifen or with pre- or postmenopausal status. 

Swerdlow AJ, Jones ME. Tamoxifen treatment for breast cancer and risk of endometrial cancer: a case-control study

The risk of endometrial cancer increases with the duration of tamoxifen treatment for breast cancer, continues even after treatment ceases, and is comparable in pre- and postmenopausal women, according to a case–control study conducted in the UK.