The most comprehensive evidence about the risks and benefits of taking hormones after menopause to prevent disease comes from the Women's Health Initiative (WHI) Hormone Program, which was sponsored by the National Heart, Lung, and Blood Institute (NHLBI) and the National Cancer Institute (NCI), parts of the National Institutes of Health (NIH). This research program examined the effects of menopausal hormones on women's health. The WHI Hormone Program involved two studies -- the use of estrogen plus progestin for women with a uterus (the Estrogen-plus-Progestin Study), and the use of estrogen alone for women without a uterus (the Estrogen-Alone Study). In both hormone therapy studies, women were randomly assigned to receive either the hormone medication being studied or the placebo.
The WHI Estrogen-plus-Progestin Study was stopped in July 2002, when investigators reported that the overall risks of estrogen plus progestin, specifically Prempro™, outweighed the benefits. The researchers found that use of this estrogen-plus-progestin pill increased the risk of breast cancer, heart disease, stroke, blood clots, and urinary incontinence. However, the risk of colorectal cancer and hip fractures was lower among women using estrogen plus progestin than among those taking the placebo. In addition, the WHI Memory Study showed that estrogen plus progestin doubled the risk for developing dementia (a decline in mental ability in which the patient can no longer function independently on a day-to-day basis) in postmenopausal women age 65 and older. The risk increased for all types of dementia, including Alzheimer's disease.
The WHI Estrogen-Alone Study, which involved Premarin™, was stopped in February 2004, when the researchers concluded that estrogen alone increased the risk of stroke and blood clots. In contrast with the WHI Estrogen-plus-Progestin Study, the risk of breast cancer was decreased in women using estrogen alone compared with those taking the placebo. Use of estrogen alone did not increase or decrease the risk of colorectal cancer. Similar to the results seen in the Estrogen-plus-Progestin Study, women using estrogen alone had an increased risk of urinary incontinence and a decreased risk of hip fractures.
This answer is based on source information from the National Cancer Institute.