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NIH Research Matters

April 28, 2006

New Analysis of Estrogen Therapy and Breast Cancer Risk

Estrogen-alone hormone therapy does not increase the risk of breast cancer in postmenopausal women, according to a new analysis of results from the Women's Health Initiative (WHI), one of the largest medical studies ever.

photo of a woman smiling

The Estrogen-Alone portion of the WHI involved 10,739 generally healthy postmenopausal women ages 50-79 who did not have a uterus. Estrogen-alone therapy (without progestin) is only recommended for women without a uterus; those with a uterus who take estrogen have an increased risk of endometrial cancer, so they are advised to take estrogen combined with progestin.

The study was stopped at the end of February 2004 because of an increased risk of stroke and no significant effect on heart disease. Other findings from the study at the time were that estrogen increased the risk of blood clots in the legs, reduced the risk of hip fractures and had no significant effect on colorectal cancer.

The effect of estrogen on breast cancer was uncertain. This new report, published in the April 12 issue of the Journal of the American Medical Association, provides a more detailed analysis of 237 invasive breast cancers.

Over an average of about 7 years of follow-up, the women taking estrogen had fewer breast cancer tumors than those in the placebo group, but the difference between the two groups was not statistically significant — in other words, it could have happened by chance. These results contrast with results from the WHI's Estrogen plus Progestin Trial, which found an increase in breast cancer over about 5 years among those taking the combined hormone therapy. The new analysis did find, however, that participants taking estrogen had 50% more abnormal mammograms that required follow-up and underwent 33% more breast biopsies.

Dr. Elizabeth G. Nabel, Director of NIH's National Heart, Lung, and Blood Institute, which sponsored the study, commented, "The findings still support current recommendations that hormone therapy should only be used to treat menopausal symptoms and should be used at the smallest effective dose for the shortest possible time."

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Editor: Harrison Wein, Ph.D.
Assistant Editors: Vicki Contie, Carol Torgan, Ph.D.

NIH Research Matters is a weekly update of NIH research highlights from the Office of Communications and Public Liaison, Office of the Director, National Institutes of Health.

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This page last reviewed on December 3, 2012

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