Breast cancer risk associated with the use of hormone therapy is much smaller than most women have been led to believe, according to researchers in Australia who have calculated the risk in a new way.

Widely reported studies suggest that compared to nonhormone users, women who take hormones for many years have anywhere from a 50 percent higher risk of developing breast cancer to double the risk.

While that sounds pretty scary, the new research indicates that the real risk is actually quite small, says John Boyages, MD, who is executive director of the New South Wales Breast Cancer Institute.

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Absolute Risk Low

Boyages and colleagues used a mathematical formula to estimate the excess risk of breast cancer due to exposure from hormone therapy use over time. This is known as the cumulative absolute risk.

They found only a slightly higher risk in women who took a combination of estrogen and progestin for up to five years.

Far from doubling the risk, the researchers suggest that the risk in a 50-year-old woman increased from 6.1 percent to 6.7 percent if she took combined hormones for five years. The risk for the same woman who took hormones for a decade was estimated to be 7.7 percent.

The findings are reported in the Aug. 6 issue of the British Medical Journal.

"We are not saying that there is no risk associated with taking hormones," Boyages tells WebMD. "But I do think that women have been unnecessarily scared by the figures they have seen in the media. We have to work harder to communicate risk in ways that women can understand."

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Measuring Risk

Up until three years ago, hormone therapy was widely prescribed to menopausal women in the belief that the treatment would protect against heart disease and other conditions associated with aging.

The early halting of the U.S. government-sponsored Women's Health Initiative prevention trial in July of 2002 caused millions of women to abandon the treatment. Researchers initially reported a "small but unacceptable" increase in breast cancers associated with hormone therapy, as well as increases in heart attacks, strokes, and blood clots.

Hormone therapy is indicated for the treatment of moderate to severe hot flashes and other symptoms associated with menopause such as vaginal dryness and the prevention of postmenopausal osteoporosis.

North American Menopause Society executive director Wulf Utian, MD, PhD, points out that the WHI findings, like those from the Australian study, show that the absolute breast cancer risk associated with hormone therapy use is relatively small.

Thirty-eight WHI participants out of 10,000 who took estrogen plus progestin for more than five years developed breast cancer each year, compared to 30 women out of 10,000 who never took hormones.

"I think women can understand an extra eight cases a year per 10,000," Utian tells WebMD, adding that the data are usually presented in ways that are more frightening.

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Lowest Dosage, Shortest Time

Boyages, who is an oncologist, says he became aware of the need for finding better ways to communicate risk to women after realizing that his patients had a poor understanding of their individual risk.

"As a breast cancer doctor I have seen many patients who were certain that taking hormones caused their cancer," he says. "For most women that was simply not the case. We need better tools to communicate the real risk so that women can make informed decisions about the use of hormone therapy."

The North American Menopause Society and other health groups now recommend that women take hormones for menopausal symptoms only at the lowest effective dosage for the shortest time needed.

Boyages tells WebMD that the breast cancer risk associated with hormone therapy can be reduced if women use the treatment in this way.

"Use for five years or less seems to be reasonably safe," he says. "After that the risk starts creeping up."

By Salynn Boyles, reviewed by Brunilda Nazario, MD

SOURCES: Coombs, N.J. British Medical Journal, Aug. 6, 2005; vol 331: pp 347-349. John Boyages, MD, oncologist; executive director, New South Wales Breast Cancer Institute. Wulf Utian, MD, PhD, executive director, North American Menopause Society; professor of gynecology, Case Western Reserve University, Cleveland.