The Women's Health Initiative found that women taking the combination estrogen-progestin (Prempro) used in the study had an increased risk of developing certain serious conditions. According to the study, over one year, 10,000 women taking estrogen plus progestin compared with a placebo might experience:
- Seven more cases of heart disease
- Eight more cases of breast cancer
- Eight more cases of stroke
- 18 more cases of blood clots
Based on these numbers, the increased risk of disease to an individual woman is small. However, the overall risk to menopausal women as a group became a substantial public health concern. In addition, researchers found that women taking combination estrogen-progestin had an increase in abnormal mammograms. The higher number of false-positives — signs of possible breast cancer that ultimately prove inaccurate — was probably due to estrogen, which increases breast tissue density.
For women taking estrogen alone (Premarin), the WHI found no increased risk of breast cancer or heart disease. But researchers did find that over one year, 10,000 women taking estrogen compared with a placebo might experience 12 more cases of stroke and six more cases of blood clots in the legs, plus an increase in mammography abnormalities. This last point is important, because women who take estrogen or combination estrogen-progestin therapy may need more frequent mammograms and additional testing.