LOS ANGELES — Many women who used estrogen alone as hormone replacement therapy after menopause had a lower risk of developing breast cancer up to five years after they stopped taking it, a study has found.
The research, published Tuesday, adds another twist to the evolving story on whether hormone replacement therapy helps some women beyond treating menopausal symptoms such as hot flashes and poor sleep quality.
The report is a follow-up analysis of the landmark Women's Health Initiative, a clinical trial of tens of thousands of women begun in 1993 that sought to clarify the risks and benefits of two hormone replacement therapy regimens in postmenopausal women: estrogen plus progestin, which most women must take, and estrogen alone, taken by women who had hysterectomies.
The double-hormone arm of the study was abruptly halted in 2002 after scientists found that it raised the risk of breast cancer without conferring hoped-for benefits on the heart. In 2004, the estrogen-alone arm of the study was also halted after researchers discovered an increased risk of stroke and blood clots.
At the time, it was not clear how estrogen alone affected breast cancer risk. But research since then found that estrogen alone did not increase risk and maybe even lowered it.
The new study, published in the journal Lancet Oncology, provides the strongest evidence yet that estrogen alone not only lowers breast cancer risk for some women but curbs the chances of dying from the disease.
"It's a very interesting finding," said a co-author of the study, Dr. Rowan Chlebowski, an investigator at the Los Angeles Biomedical Research Institute and chief of medical oncology and hematology at Harbor-UCLA Medical Center.
Researchers followed 7,645 women from the original group of almost 11,000 participants for almost five years to see what happened to them after stopping estrogen therapy. The study found that women who took estrogen had a 23 percent reduced risk of breast cancer compared with those who took a placebo.
Among the women who did develop breast cancer, those who took estrogen had a 63 percent reduced risk of dying from the disease compared with those who took a placebo.
There are nuances to the findings, however. The lowered risk of breast cancer was striking among women who had a typical breast cancer risk. In women classified as at higher risk of breast cancer — either because of family history or a medical history of benign breast tumors — estrogen did not lower the risk.