Premarin is a brand name for conjugated estrogens used to treat menopausal symptoms and postmenopausal osteoporosis. It is manufactured by Wyeth Pharmaceuticals, a subsidiary of Pfizer.
The drug's active ingredient is a mixture of natural estrogens obtained from pregnant mares' urine (hence the Premarin name). These estrogens are structurally similar to human estradiol, the predominant form of estrogen in women during childbearing years.
The FDA first approved Premarin in 1942 to treat menopausal symptoms such as hot flashes and vaginal dryness. In 2003, the FDA also approved it for the prevention of postmenopausal osteoporosis.
Today, Premarin remains one of the most popularly prescribed drugs in its class, with over 10 million prescriptions filled annually in the United States alone.