Premarin:  A synthetic estrogen made from horse urine.  Human females function on three types of estrogen:  estriol (90%), estrone (3%) and estradiol (7%).  Premarin contains no estriol but has disproportionate amounts of estrone (75-80%) and estradiol plus three equine estrogens found exclusively in horses.  A woman's body contains all the necessary enzymes and co-factors to metabolize and process estriol, estrone and estradiol in their naturally occuring proportions; but lacks any means to process the equine (horse) estrogens.  This may contribute to the unatural feelings and untoward side effects many women experience on Premarin.