Women who take estrogen after menopause appear to have a lower risk of breast cancer even years after they quit taking the hormone, according to a new analysis of a landmark study.
The results are reassuring news for women who have had hysterectomies and use the pills to relieve hot flashes and other symptoms of menopause, the researchers and other doctors say. Previous observational studies have suggested a possible connection between estrogen and breast cancer.
The new research, published in the journal Lancet Oncology, found women who had a hysterectomy who took estrogen-only pills for about six years were about 20 per cent less likely to develop breast cancer than those who didn't take the hormone, and the benefit lasted for at least five years.
"If women are suffering from serious menopause symptoms and have had a hysterectomy, then estrogen alone is a reasonable approach," said the study's lead author, Garnet Anderson of the Fred Hutchinson Cancer Research Center in Seattle.
Doctors have long prescribed hormones for women after menopause to relieve symptoms like hot flashes and night sweats. The pills were also believed to be good for bones, the heart and have other health benefits.
In the 1990s, researchers began a large, U.S. funded study, known as the Women's Health Initiative, looking 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.
Those results shook up conventional wisdom about hormone replacement therapies and led women to stop taking them 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 per cent of women in menopause who have had hysterectomies. Other women are prescribed the combo pill: estrogen alone can raise their risk of cancer of the uterus.
In the new analysis, Dr. Anderson and colleagues tracked more than 7,600 postmenopausal women aged 50 to 79 who had a hysterectomy. Roughly half took estrogen while the other half took placebo pills for about six years. Most women in both groups had yearly mammograms. The women were followed for about 12 years.
In the group that took estrogen, there were 151 cases of breast cancer versus 199 in those on fake pills. That amounted to a 23 per cent lower risk of cancer, researchers said.
In women who developed breast cancer, there were six deaths among those who had taken estrogen compared to 16 in those who took placebos. The lower risk of breast cancer didn't apply to women with a family history of the disease or those who previously had benign breast lumps.
Doctors said women should not take estrogen to lower their breast cancer risk since the hormone comes with slightly higher chances of stroke and blood clots. Research published last year found those problems appeared to fade after women stopped taking the pills.
"Estrogen on its own appears to be safe," said Anthony Howell, professor of medical oncology at the University of Manchester, who co-authored a commentary in the journal.
Scientists aren't sure why estrogen appeared to lower the risk of breast cancer, but Dr. Howell said altering the amount of estrogen in the body might help stop tumour growth, since fluctuating levels could interfere with tumour 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."