Maybe not all estroven, but Premarin stands for PREgnant MARes' urINe. I don't know if this is always the case, but I read too many horror stories about pregnant horses being kept in cages and not allowed to roam free so that their urine could be gathered.

Also (and keep in mind this is taken from Wikipedia ... not the most trustworthy source): "It is suggested that Premarin be used for the shortest period of time and at the smallest possible dose that is effective in alleviating symptoms because it can increase the risk of endometrial and breast cancers, heart attack, stroke, blood clots, and dementia."

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Linda