The WHI Estrogen-plus-Progestin study concluded that estrogen plus progestin increases the risk of invasive breast cancer. After 5 years of follow-up, women taking these hormones had a 24 percent increase in breast cancer risk compared with women taking the placebo. The increase amounted to an additional 8 cases of breast cancer for every 10,000 women taking estrogen plus progestin for 1 year compared with 10,000 women taking the placebo.
A detailed analysis of data from the WHI Estrogen-plus-Progestin study showed that, among women taking estrogen plus progestin, the breast cancers were slightly larger and diagnosed at more advanced stages compared with breast cancers in women taking the placebo. Among women taking estrogen plus progestin, 25.4 percent of the cancers had spread outside the breast to nearby organs or lymph nodes compared with 16.0 percent among nonusers. Women taking estrogen plus progestin also had more abnormal mammograms (breast x-rays that require additional evaluation) than the women taking the placebo.
The WHI Estrogen-Alone study concluded that taking estrogen did not increase the risk of breast cancer in women with a prior hysterectomy, at least for the 7 years of follow-up in the study. Further analysis of data from the study indicated a 20 percent decrease in risk of breast cancer in women taking estrogen alone, although this decrease was seen mainly in the occurrence of early-stage breast cancer and ductal breast cancer (a specific type that begins in the lining of the milk ducts in the breast). The observed reduction amounted to 6 fewer cases of breast cancer for every 10,000 women taking estrogen for 1 year compared with 10,000 nonusers, but this lower incidence was not statistically significant, i.e., it was less than what might be expected to happen by chance alone. The Estrogen-Alone study also showed a substantial increase in the frequency of abnormal mammograms.
A comprehensive review of data from 51 epidemiological (population) studies published in the 1980s and 1990s found a statistically significant increase in breast cancer risk among current or recent users of any hormone replacement therapy compared with the risk among nonusers. Most women in the analysis (88 percent) had used estrogen alone, and data for estrogen-plus-progestin users was not analyzed separately. Analysis of the pooled data also showed that the risk of breast cancer increased with increasing duration of hormone use, and this effect was more prominent in women with low body weight or a low body mass index. However, breast cancers in hormone users were less likely to have spread to other parts of the body compared with the breast cancers in nonusers. The increase in breast cancer risk largely, if not completely, disappeared about 5 years after cessation of hormone use.
This information is produced and provided by the National Cancer Institute (NCI). References are available at their website at www.cancer.gov. The information in this topic may have changed since it was written. For the most current information, contact the National Cancer Institute via www.cancer.gov or call 1-800-4-CANCER.