When initiated soon after menopause, hormone therapy with estradiol prevented degeneration in key brain regions of women who were at heightened dementia risk, according to a new study. The investigators also found that another type of hormone therapy, marketed under the brand name Premarin, was far less protective. Premarin is a mixture of 30-plus substances derived from the urine of pregnant mares. Estradiol -- the dominant sex-steroid hormone in woman -- accounts for about 17 percent of Premarin's total content. Other Premarin components exert various endocrinological effects on different tissues.