Historically, progesterone (eg Natpro) and synthetic progestins have been lumped together with respect to their safety profiles, although they are very different in their molecular structure and effects.[This is discussed in the section on Bioidentical (Natural) Progesterone vs Synthetic Progestins.]
There is no documented evidence in the scientific literature of any cases of cancer as a result of treatment with bioidentical progesterone. Unfortunately, progesterone has been implicated in the development of breast cancer because of the results of large trials in which an increase in the incidence of breast cancer was seen when synthetic progestins were used in combination with estrogens for postmenopausal hormone therapy. These studies, such as the Women’s Health Initiative, DID NOT use bioidentical progesterone.
A large European study, on the other hand, found that women using bioidentical progesterone plus estrogen had a lower risk of breast cancer than women using estrogen alone, whereas women using synthetic progestins plus estrogen had a significantly higher risk of breast cancer. [See the section on Progesterone and Breast Health for the research on progesterone and breast cancer.]
With thanks to http://www.womeninbalance.org