In a study (accepted for publication in May 2006, in Menopause: The Journal of The North American Menopause Society) investigators found the the use of progesterone increased the risk of breast cancer metastases. The authors conducted this study in animals with breast cancer. They noted that progesterone’s effect was not dependent on the menopausal status of the animal nor whether the progesterone was given with or without estrogen. It appeared that progesterone stimulated the production of blood vessels to the tumors, increasing their access to the circulation and the rest of the body.
Millions of women, after menopause, take combination estrogen/progesterone hormone therapies to help them manage some of the distressful symptoms of menopause (i.e. hot flashes). It has been established (from prior human studies) that estrogen/progesterone hormonal therapies increase the risk of developing breast cancer in post-menopausal women.
Although this study is not a human study, the implications suggest that women with a higher risk of developing breast cancer should be aware of the potential increased risk of breast cancer progression (through metastastases) if they are taking a progesterone-containing hormonal therapy.
