Does taking progesterone pose an increased risk for breast cancer?

Thank you for your excellent question about progesterone and breast cancer.

Does taking progesterone pose an increased risk for breast cancer?
The short answer is NO!  But we have no randomized controlled trial data showing that (yet—some are just starting now) There has been no risk of breast cancer ever documented from taking progesterone alone and without any estrogen. Obviously the concern is that your perimenopausal system is likely making high levels of estrogen, at least intermittently now. Still your estrogen levels are now likely to start returning to normal premenopausal levels and then eventually (in two or three years) will start becoming the low that is normal for menopause.

We have been concerned about breast cancer risks from progesterone since both estrogen alone and estrogen with progestins increase breast cancer risk based on randomized controlled trial (the Women’s Health Initiative) and epidemiology (Million Women Study in Britain and the E3N study in France) data from around the world.

The only large dataset providing answers about estrogen and progesterone and breast cancer risk is from the large (~80,000) French study of women teachers (E3N) followed over more than 8 years who were menopausal and whose breast cancers were carefully documented:

• the women who did not take menopausal hormone therapy (MHT) are the controls against the rest were compared
• some, mostly who had a hysterectomy but some who also had their ovaries out, took estrogen alone—there was a 26% increased risk for breast cancer
• more took estrogen and a progestin (synthetic) of varying types—there was a 69% increased risk for breast cancer
• about 20% of those who used MHT took estrogen and progesterone—there was no (0%) increased risk for breast cancer

How does progesterone act related to breast cancer risk:

• estrogen causes cells to proliferate—if that is not controlled then cancer results
• progesterone causes proliferation to decrease and cells to become more mature—those are actions that decrease cancers
• when progesterone is present its receptors make the estrogen receptors no longer able to cause proliferation

I personally have a high risk for breast cancer because my mother had it. Yet I needed and took progesterone for 7 years for very bad hot flushes and night sweats. So you can see that the evidence says to me, personally, that progesterone is safe related to risks for breast cancer.