Additional information from the Women’s Health Initiative firmly establishes that postmenopausal hormone therapy with combined estrogen plus progestin increases a woman’s risk of developing breast cancer. According to another study, however, breast cancers that develop in women who have used estrogen plus progestin tend to be less deadly than breast cancers in other women. The results of both studies were presented at the 2008 San Antonio Breast Cancer Symposium.
Postmenopausal hormone therapy with the female hormones estrogen alone or estrogen plus progestin (combined hormone therapy) effectively manages several common menopausal symptoms. However, a large clinical trial conducted as part of the Women’s Health Initiative (WHI) raised concerns about the health risks of these therapies. In 2002, for example, it was reported that combined estrogen plus progestin increases the risk of breast cancer, heart disease, stroke, and blood clots. Women taking estrogen plus progestin had fewer fractures and were less likely to develop colorectal cancer, but for most women, these benefits were thought to be outweighed by the risks.
Researchers used updated information from the WHI clinical trial of estrogen plus progestin, as well as information from another component of the WHI (the WHI Observational Study), to further explore the relationship between estrogen plus progestin and risk of breast cancer.
- The results confirmed that users of estrogen plus progestin were more likely than nonusers to develop breast cancer. This increased risk declined markedly and fairly rapidly, however, once women stopped using hormones.
These analyses of the WHI data support the claim that the recent decreases in breast cancer incidence in the United States may be due to a reduction in the number of women using postmenopausal hormones. It also provides information to women and physicians about what happens to breast cancer risk after hormone cessation.
In a second presentation, researchers assessed the impact on breast cancer survival of hormone use prior to breast cancer diagnosis. Using information from the California Teachers Study, researchers examined breast cancer survival among 2,783 postmenopausal women with breast cancer. 
- After accounting for other factors that may influence breast cancer survival (such as stage of disease and general health), women who used estrogen plus progestin before their breast cancer diagnosis were 47% less likely to die of breast cancer. There was a suggestion that estrogen alone may also decrease risk, but the effect was smaller and not statistically significant (suggesting that it could have occurred by chance alone).
This study suggests that breast cancers that develop in users of estrogen plus progestin have a tendency to be less deadly than breast cancers that develop in women who have never used postmenopausal hormones. It must be remembered, however, that breast cancer is also more common in women who use estrogen plus progestin.
Women who are currently using postmenopausal hormones, or who are considering use, are advised to discuss the risks and benefits with their health care provider.
 Chlebowski RT, Kuller L, Anderson G et al. Breast cancer after stopping estrogen plus progestin in the Women’s Health Initiative. Presented at the San Antonio Breast Cancer Symposium. December 13, 2008. Abstract 64.
 Marshall SF, Chang E, Clarke CA et al. Hormone therapy use before diagnosis and breast cancer survival in the California Teachers Study. Presented at the San Antonio Breast Cancer Symposium. December 13, 2008. Abstract 65.