Estrogen HRT: No Breast Cancer Risk

April 11, 2006 -- After hysterectomy, estrogen-only hormone therapy does not increase a woman's risk of breast cancer.

That's the word from the final analysis of the estrogen-only arm of the Women's Health Initiative (WHI) study. The trial was stopped early because it linked hormone therapy to increased risk of stroke, with no reduction in heart-disease risk.

The study had two arms. Menopausal women with an intact uterus received estrogen plus progestin or an inactive placebo. That's because estrogen alone increases a woman's risk of uterine cancer. Women who had undergone hysterectomy, however, received estrogen-only hormone therapy or placebo.

The average five-year breast cancer risk estimates were similar between the estrogen-plus-progestin group and the estrogen-only group.

Could this be true? Stanford researcher Marcia L. Stefanick, PhD, chairwoman of the WHI steering committee, led a team that took a detailed look at the estrogen-only data. Stefanick's report appears in the April 12 issue of The Journal of the American Medical Association.

"The major finding is no increased risk of breast cancer in women taking just estrogen," Stefanick tells WebMD. "There is a tendency for decreased risk of breast cancer, but it is not statistically significant. This means it would be inappropriate simply to say estrogen alone decreases breast cancer."

Stefanick says estrogen may be safer for some women than for others.

"Women at lower initial risk of breast cancer seem to be the ones who have even lower risk if they go on estrogen," she says. "Women with a higher risk of breast cancer seem to have even higher risk with estrogen."

HRT Risk Overblown?

Stefanick says there are big differences between treating women who have had a hysterectomy and women with an intact uterus. And there are big differences between estrogen-only hormone therapy and estrogen-plus-progestin therapy.

"The media always talk about 'hormones do this' or 'hormones do that,'" Stefanick says. "What we are clear on is estrogen plus progestin does one thing in one group, and estrogen alone does something very different in a different group. ... We cannot say it is all about progestin. But we can say progestin plus estrogen gives a really different outcome than just estrogen."

The study results are very reassuring to Hugh Taylor, MD, associate chief for research in the obstetrics, gynecology, and reproductive sciences department at Yale University. Taylor spoke at a news conference organized by Wyeth, which makes Premarin and Prempro, the hormones tested in the WHI study. Taylor has received speakers' fees from Wyeth, but has no financial interest in the company. Wyeth is a WebMD sponsor.

Controversy Continues

"Women have been on a roller coaster ride. That ride is over," Taylor says. "For women with hysterectomy, the evidence is clear: Estrogen is safe for them. There is no breast cancer risk for estrogen alone. This has been the No. 1 fear keeping women from using estrogen for their menopausal symptoms."