Premarin is the brand name for an estrogen medication that consists of conjugated estrogens isolated from the urine of pregnant mares (pregnant mares' urine). It is manufactured by Wyeth (now a part of Pfizer), and was first marketed in 1941 and 1942 in Canada and the United States, respectively, for the treatment of menopausal symptoms.Premarin is available in oral (0.3 mg, 0.625 mg, 0.9 mg, 1.25 mg, or 2.5 mg pills), intravenous, and vaginal (cream) forms.