Progesterone is a naturally occurring hormone in women. Progesterone is added to estrogen therapy to reduce the risk of endometrial cancer. Initially, progesterone was not available in a form that could be well absorbed, so pharmaceutical manufacturers created a synthetic progestin (progesterone derivative), medroxyprogesterone acetate. Now, progesterone is available in a micronized form that is better absorbed; allowing women to receive bioidentical progesterone instead of a synthetic progestin as part of their hormone therapy. While both progesterone and synthetic progestins protect the endometrium, there are significant differences in their effect on breast tissue: progesterone induces death of breast cancer cells while progestins may significantly increase estrogen-stimulated breast cell proliferation, which can contribute to the development of breast cancer.
In a study of over 80,000 postmenopausal women, the following breast cancer risk was observed:
- Women who used estrogen only were found to have a 29% increased risk of breast cancer compared to women who had never used Hormone Replacement Therapy (HRT)
- Women who used estrogen plus a synthetic progestin were found to have a 69% increased risk of breast cancer compared to women who had never used HRT
- Women who used estrogen plus progesterone were found to have no increased risk for breast cancer
“These findings suggest that the choice of the progestagen component in combined HRT is of importance regarding breast cancer risk; it could be preferable to use progesterone.” (Progestagen – also spelled progestogen – is the class of hormones that includes progesterone and progestins.)
Breast Cancer Res Treat. 2008; 107(1):103-111.
The Developmental Endocrinology Branch, National Institutes of Health, had previously shown that the addition of natural testosterone (not METHYLtestosterone) to conventional hormone therapy for postmenopausal women does not increase and may indeed reduce the risk of breast cancer.
Menopause. 2004 Sep-Oct;11(5):531-5.
A study of the influence of topically administered estradiol and progesterone on human breast tissue concluded that progesterone administration decreases estrogen-induced breast cell proliferation by 400%.
Fertil Steril. 1995;63(4): 785-791.