Premarin is used alone for post-menopausal women without a uterus.
PremPro is used for post-menopausal women with a uterus.
Premarin is short for “Pregnant Mares’ Urine”*. Premarin is a Conjugated Estrogen, a mixture of estradiol and several minor estrogens. Estradiol is the active hormone; the others are significantly less potent and unnecessary. Furthermore, many of these estrogens in Premarin are unique to horses. Horse Estrogens no longer have a role in treating humans when there are available purer compounds identical to the hormones made by pre-menopausal women.
*(Premarin is farmed from pregnant mares housed in massive, crampt wharehouses (mainly in Alberta, Canada). Horses are bred, not for their speed or beauty, but for their ability to produce estrogen-rich urine. Thousands of foals and older mares no longer able to conceive are starved to death or destroyed and sold for horsemeat in Europe.)
PremPro is a combination pill: Premarin + medroxyprogesterone (Provera). Medroxyprogesterone is a synthetic progesterone with side effects that naturally occurring progesterone does not have.