For women in their 70s, however, estrogen-only HRT increased risk of colorectal cancer, chronic disease, and death, according to seven-year follow-up data from the Women's Health Initiative (WHI).
The findings do not mean that women should take estrogen to prevent breast cancer. But they do suggest that some younger postmenopausal women -- those with a prior hysterectomy -- may take estrogen for up to six years without significant risk.
"We are not arguing that women should use estrogen to prevent breast cancer," study researcher Andrea Z. LaCroix, PhD, of Seattle's Fred Hutchinson Cancer Research Center, tells WebMD. "What we are saying is there are these important risks and benefits to estrogen-only HRT. Now women and their doctors have more information than ever before on deciding whether to start estrogen and when to stop it."
The WHI is the study that in 2004 showed that HRT given during or after menopause did not, as expected, cut a woman's risk of heart disease -- but did increase risk of stroke and dangerous blood clots.
The findings revolutionized health care for older women, ending routine use of hormone replacement except as a treatment for severe menopausal symptoms. "As little as possible for as little time as possible" became the mantra for patients receiving hormone therapy.
Early on, it became apparent that combined estrogen/progesterone HRT was riskier than estrogen-only HRT. But estrogen-only HRT can be taken only by women who have had a hysterectomy, as unbalanced estrogen greatly increases the risk of uterine cancer. And it, too, appeared to carry serious risks.
Now estrogen-only HRT for younger postmenopausal women -- for up to six years -- appears to carry fewer long-term risks, and to offer greater benefits, than previously thought.
"For coronary heart disease, heart attack, colorectal cancer, death, and a global index of chronic disease, the women in their 50s on estrogen alone have lower risk of these conditions than women taking a placebo," LaCroix says. "For women in their 70s it had the opposite effect. They had increased risk for all these things on estrogen alone."
In an editorial accompanying the LaCroix report in the April 6 issue of the Journal of the American Medical Association, Washington University researchers Emily S. Jungheim, MD, and Graham A. Colditz, MD, DrPH, have a far different opinion of the new WHI results.
They note that the WHI was designed to see whether HRT protected against heart disease. Today, HRT is used to treat symptoms of menopause -- but the women in the WHI were not taking HRT for this reason.
"So are women experiencing severe menopausal symptoms the same as the women who enrolled in the WHI? I don't think so," Jungheim tells WebMD. "And the estrogen prescribed today may be different than the Premarin used in the WHI. It is hard to inform patients based on this data."
LaCroix disagrees, and notes that there never will be large-scale clinical trials of modern forms of HRT.
"All we can say is the best evidence on HRT comes from this large trial," she says. "We got very reassuring data for women in their 50s who can take estrogen-only HRT. I don't have any evidence that 'more natural forms' of estrogen are safer or have different risk profiles. They must be assumed equal unless there is new evidence."
While LaCroix and Jungheim disagree strongly about the relevance of the WHI findings, they agree that women facing severe menopausal symptoms should have a long, frank talk with their doctors.
"Every woman has to make her own choices," LaCroix says. "If you are having bothersome menopausal symptoms, you have some options."
"This is an opportunity for women to sit down with their doctors and be honest about the menopausal symptoms they are experiencing, and for these doctors and patients to come up with a plan so the woman is functioning at her best," Jungheim says. "That plan will vary from woman to woman depending on what her risk factors are for something else later on in life, and on what she has already tried and is eligible to try based on her risk profiles."