But, progesterone cannot be patented and so the company that makes Premarin came up with a Progestin, as Katie pointed out, and they called it Provera
is a relative of this). The two drugs together, Premarin and Provera, Equal Prempro. It is now theorized that the Progestins are what helps create the big mess in the synthetic world of hormones. The snythetic estrogens are not great either ( I, like you, would only use a bio estrogen like Vivelle) but that Progestin is nasty.