Hormone replacement therapy (HRT) was part of a large set of clinical trials called the Women's Health Initiative (WHI). The HRT portion of the study was ended 3 years early because of a small but unacceptable increase in the rate of breast cancer among the women taking a combination of 0.625 mg of estrogen plus 2.5 mg of progestin daily. As the WHI data has been analyzed, risks of blood clots, heart disease, ovarian cancer, and dementia have also been identified. Experts do not yet know whether lower-dose, shorter-term HRT reduces or eliminates these risks.

The results of the WHI study can be hard to understand. Some of the women in this study took HRT (estrogen and progestin) and some took ERT (estrogen only). Each group was compared to women who took a placebo.

The study found that in a small number of women, using HRT increased the risk of:¹

• Breast cancer.
• Stroke.
• Heart disease.
• Blood clots.
• Dementia in women 65 and older.

The WHI study also found that HRT may reduce the risk of:¹

• Colorectal cancer.
• The number of broken bones in women who are past menopause.

The WHI study also found that in a small number of women, using ERT increased the risk of:¹

• Stroke.
• Blood clots.

The WHI study found that women who took ERT had fewer broken bones than women who did not take ERT. The study also found that women who took ERT did not have an increased risk of heart disease or colorectal cancer.¹ Follow-up studies have shown that a woman's age when she started taking ERT affects whether she has an increased risk of heart problems and colorectal cancer. The reduced risk of hip fracture did not continue after ERT was stopped.²

The link between ERT and breast cancer is unclear. One study showed that the risk of breast cancer was lower for women who took ERT even after they stopped taking it.²

For women who took ERT, the study found no increase in the risk for heart attack or the risk for colorectal cancer. The risk of breast cancer for women taking ERT is unclear. But women who took ERT also had fewer broken bones than women who did not take ERT.¹

It is important to remember that average risks from HRT and ERT are low among the general population of women. But your personal risk that hormone therapy may lead to certain health problems, such as heart disease or breast cancer, may be lower or higher depending on your risk factors for those health problems.

For more information about the Women's Health Initiative study, see the U.S. National Institutes of Health Web site about the WHI at www.nhlbi.nih.gov/whi.