Despite the category it is filed under, Premarin is an oestrogen rather than an anti-oestrogen: a preparation of conjugated oestrogens, chiefly sodium estrone sulphate together with equine oestrogen sulphates, originally isolated from pregnant mares' urine, which is where the contracted name comes from.
The tablets are 0.625 mg and come as a 28-tablet pack. Pfizer inherited the brand through its acquisition of Wyeth, and the product is one of the oldest hormone preparations still in continuous manufacture. In medicine it is prescribed for menopausal symptoms and other oestrogen-deficiency states.
