source: http://womeninbalance.org/resources-research/bioidentical-progesterone-vs-synthetic-progestins/

Confusion exists, even among leading experts in the field of hormone research, between the terms progesterone, progestin, and progestogen. Although these terms are often used interchangeably, they are not synonymous.

Until an authoritative definition is formally adopted, the following reflects the current culture of the usage of these terms, and clarifies how they are used by Women in Balance: “progesterone” refers to the hormone produced in the body, or produced from a plant source but still chemically and structurally identical to human progesterone, and it is therefore referred to as “bioidentical” or “natural”.

In contrast, “progestin” refers to a hormone that is synthetically produced and differs in structure from progesterone. There are numerous synthetic progestins used in hormone therapy, in contrast to only one molecule referred to as progesterone. “Progestogen” (sometimes spelled “progestagen”) is a general term for hormones that act like progesterone in the uterus, and therefore includes both progesterone and progestins.

There is increasing evidence that, by virtue of their different chemical structures, synthetic progestins do not always act as progesterone would at the same target tissues. This has long been understood with respect to treatment of pregnancy and fertility issues, when progesterone is effectively prescribed, yet synthetic progestins are contraindicated.

While synthetic progestins may mimic some of progesterone’s effects, progestins may react differently with progesterone receptors in the body. A significant consequence of the side effects seen with synthetic progestins has been an increase in the risk of developing breast cancer. Clinical trials such as the Women’s Health Initiative, in which more breast cancer was seen in the group taking progestins, did not study natural progesterone