Researchers said the findings are good news because they add to evidence that the benefits of hormone supplements outweigh any increased risk of breast cancer. The study appears in Wednesday's Journal of the American Medical Association.
In fact, 90 percent of breast cancers are a type that this study found hormone replacement therapy had no effect on, reports CBS News Health Correspondent Dr. Emily Senay.
According to the study, there was no evidence that taking hormones was associated with the most common, but more deadly forms of the disease.
Millions of women take estrogen to ease the symptoms of menopause, such as hot flashes. The hormones are also known to reduce the risk of heart disease, brittle bones and possibly even mental decline.
Previous studies had indicated that women who take estrogen after menopause are more likely to develop breast cancer. Other studies have found no link.
Since millions of post-menopausal women every year make the decision whether to take hormone replacement therapy, the study gives them more information to consider. A woman with a strong family history of osteoporosis and the moderate risk of breast cancer may consider starting the therapy because she knows that will help reduce her high risk of getting brittle bones, Dr. Senay says.
This was the first hormone study to categorize cases of breast tumors according to whether they were slow-growing or fast-growing. "When a woman weighs the risk of breast cancer vs. the benefit of possibly reducing cardiovascular disease and reducing risk of osteoporosis, this just provides further evidence for the benefit," Dr. Susan M. Gapstur told the Associated Press. Gapstur, a cancer epidemiologist at Northwestern University Medical School in Chicago, led the study.
It may help explain the conflicting data in other studies and why hormone takers survive breast cancer more often than non-takers. "One possible explanation is that they develop more favorable tumor types," said Dr. Melody A. Cobleigh, director of the Comprehensive Breast Center at Rush-Presbyterian-St. Luke's Medical Center in Chicago. She was not involved in the study.
The researchers analyzed data on 37,105 subjects enrolled at ages 55 to 69 in the Iowa Women's Health Study. Over 11 years ending in 1996, a total of 1,520 cases of breast cancer developed among the women.
Women who took hormones had no difference in their risk of getting the fast-growing, life-threatening tumors that make up 85 percent to 90 percent of all cases of breast cancer than women who didn't take hormones.
Women who used hormones for five years or less were 1.8 times more likely to have slow-growing, highly curable tumors than women who never took hormones. Those who had use hormones for more than five years were 2.6 times more likely to have the least-threatening tumors.