The study's researchers and other doctors say the news is reassuring for women who have had hysterectomies and take the pills to relieve hot flashes and other symptoms of menopause.
The study focused on women who had a hysterectomy and took estrogen-only pills for about six years. According to the results, these women were 20 percent less likely to develop breast cancer than those who didn't take the hormone, and the benefits lasted at least five years.
Doctors have long prescribed hormones for women after menopause to lessen symptoms like hot flashes and night sweats. The pills were also believed to be helpful for bones, the heart and carry other health benefits.
In the 1990s, researchers launched a large U.S.-funded study to look at the effects of estrogen-progestin combination pills and estrogen-only therapies.
The estrogen-progestin part of the study was stopped in 2002 when the combo pill was linked to higher risks for heart attacks and breast cancer. In 2004, the estrogen study was halted after researchers detected stroke and blood clot risks in that group.
The study led to women stopping hormone replacement therapies in droves. Now the advice is to take the hormones to relieve symptoms at the lowest dose possible for the shortest amount of time because of the potential risks.
Estrogen-only pills are recommended for the approximately 25 percent of women in menopause who have had hysterectomies. Other women are prescribed the combo pill, since estrogen alone can raise their risk of cancer of the uterus.
Although the results of the study highlights the benefits of estrogen, doctors say women should not take the hormone just to lower their breast cancer risk since it comes with slightly higher chances of stroke and blood clots.
Researchers were not sure why the estrogen appeared to lower breast cancer risk, but medical experts say altering the amount of estrogen in the body might help stop tumor growth, since fluctuating levels could interfere with tumor development.
Other experts weren't convinced. "It's inconsistent with the totality of evidence that finds estrogen increases breast cancer risk," said Valerie Beral, director of the cancer epidemiology unit at Oxford University. She said the analysis was a subset of a larger trial that wasn't designed to specifically look at breast cancer.
"If you want to take hormone replacement therapy, estrogen-only has a much lesser effect on breast cancer than with progestin," she said. "But to say it protects against breast cancer is wrong."
On the other hand, other professionals say the study was still reassuring news for women who had hysterectomies seeking relief from menopausal symptoms.
The Associated Press contributed to this report.