In a study likely to cause concern among millions of women who use estrogen after menopause, Harvard researchers said today that long-term use of the hormone raised the risk of breast cancer.
Taking estrogen for less than five years, however, showed no added risk.
The study, published in the New England Journal of Medicine, comes just six months after another major study found that estrogen plays an important role in reducing the risk of heart disease in older women.
Immediately, the new research sparked controversy. Medical experts who believe that most older women should take the hormone long-term to prevent both heart attacks and osteoporosis questioned the findings. However, the study’s principal investigator, Graham Colditz of Harvard Medical School, said the findings were cause for women to reconsider estrogen’s risks versus its benefits.
“There are many ways (other than estrogen use) to lower the risk of heart disease,” Colditz said. “We don’t have many ways to lower the risk of breast cancer.”
The new study based its findings on the estrogen use and breast cancer experiences of 121,700 women participating in the long-running Nurse’s Health Study, making it one of the largest research projects to explore the hormone-cancer link.
The study is also one of very few to evaluate whether post-menopausal women who take the hormone progestin in addition to estrogen for many years are at higher or lower risk of developing breast cancer than estrogen-only users. It found that among both groups, the risk of breast cancer was roughly the same.
Among the study’s major findings:
Women taking estrogen or estrogen with progestin for five or more years and still on the hormone were at 46 percent higher risk of developing breast cancer than non-hormone users.
The risk varied depending on the woman’s age and was greatest among current long-term hormone users over the age of 60. They had about a 70 percent greater chance of developing breast cancer than nonusers. For example, a 60-year-old woman taking estrogen for at least five years had a 3 percent chance of developing breast cancer over the next five years if she kept taking the treatment. If she had never used hormones after menopause, her risk over the same period would be 1.8 percent.
Women who had taken hormones in the past, even for five years or longer, but had stopped, were not at a significantly greater risk of developing breast cancer.
Women who had taken estrogen or estrogen with progestin for less than five years were not at increased risk of breast cancer.
The latter finding is especially significant to the average older woman taking estrogen. Typically, women use the hormone for less than three years to relieve hot flashes, vaginal dryness and other menopausal symptoms as their natural estrogen levels drop in menopause.
“I think this data is reassuring to women taking estrogen to relieve menopausal symptoms,” said Colditz. “They can look at this and say, it’s basically safe to take the hormone for up to five years.”
Increasingly, though, doctors are recommending that women take hormones post-menopausally for many years, even for the rest of their lives. That is because of studies showing that estrogen lowers an older woman’s risk of heart disease by 50 percent, while at the same time preventing the bone loss that leads to the brittle bone disease called osteoporosis.
One of the chief researchers for the estrogen-heart disease study, epidemiologist Trudy Bush of the University of Maryland, was among those questioning the significance of today’s study. It provides no proof that it was the estrogen replacement therapy alone that actually caused breast cancer, she noted. “ERT has been available for over 50 years; it would have been obvious by now (if it caused breast cancer),” she said.
She added, “For a woman 50 or 60 years of age, her risk of dying from heart disease is six times greater than dying from breast cancer.”