Combining all controlled studies shows that taking estrogen, without also taking progesterone, increases a woman's chances of getting breast cancer by 26 per 100,000 cases. However, this slight increases in risk could be caused by the fact that women who take estrogen are far more likely to get mammograms which often find breast lumps before a woman can feel them. A study from the University of Saskatchewan shows that women who take progesterone with estrogen are not at increased risk for breast cancer. Taking estrogen without progesterone increases a woman's chances of suffering from uterine cancer. Estrogen stimulates the inner lining of the uterus to grow.
Progesterone stops the stimulation.
Women who are given estrogen without progesterone have uteri that are stimulated all the time, which can lead to uncontrolled growth which is cancer. However, gynecologists do not yet have the data to say the same thing for breast cancer. A recent article in the Journal of the American Medical Association recommends that clinical trials be started to find out if postmenopausal estrogen replacement can be given to women who already have breast cancer. The authors show that there is no good evidence that estrogen replacement therapy spreads breast cancer. The original data showing that postmenopausal estrogen may cause breast cancer were done in Europe on women who were given a very potent type of estrogen that has not been available in the United States.