The cancer question: an overview of recent epidemiologic and retrospective data.

Unopposed estrogen stimulates mitotic activity in endometrial and breast tissue. Numerous case-control studies have evaluated the relationship between estrogen use and the risk of endometrial and breast cancers. In general, exogenous, unopposed estrogen use increases the risk of endometrial cancer by tenfold and of breast cancer by twofold after long-term use of high doses. Estrogen's positive effects on osteoporosis and coronary heart disease must be considered when evaluating the potential risks associated with its use in postmenopausal women.