Breast cancer risk falls sharply when women stop combined hormone replacement therapy (HRT), new evidence suggests.

The research in Thursday's New England Journal of Medicine confirms that the longer a woman takes combined estrogen-progesterone, the higher her risk of breast cancer.

Furthermore, stopping HRT led to a 28 per cent drop in the risk of breast cancer within one year, Prof. Marcia Stefanick of Stanford University and her colleagues found.

The team looked at data from 41,449 women who joined a 1994 study and were free to choose hormone therapy or not, as well as more than 15,000 women participating in a clinical trial known as the Women's Health Initiative.

The Women's Health Initiative was stopped ahead of schedule in 2002 after researchers concluded that combined HRT increased the risk of heart disease, stroke and breast cancer, compared with a placebo. (The average age of women in the 2002 study was 62, which is older than when most women begin HRT.)

After the 2002 study, many women stopped HRT and sales of the hormone drugs plummeted. A few years later, breast cancer rates also dropped.

Differing interpretations

Some experts considered that decline as evidence that HRT spurs the disease while others argued that increased mammogram screening was responsible for the drop in breast cancer diagnosis. The latest study suggest those in the latter category were wrong.

"The increased risk of breast cancer associated with the use of estrogen plus progestin [the synthetic version of progesterone] declined markedly soon after discontinuation of combined hormone therapy and was unrelated to changes in frequency of mammography," the study's authors concluded.

"The difference in frequency of mammography use of 2 per cent between 2002 and 2003 for women using hormones is insufficient to account for the 43 per cent reduction in the incidence of breast cancer."

But the decline in breast cancer rates started at least three years before the Women's Health Initiative was stopped, according to the International Menopause Society, which represents HRT specialists.

"Breast cancer takes years to develop, and, to reach the stage where it is detectable, it takes at least a decade," the group said in a statement. "If HRT use causes breast cancer, then the drop in breast cancer rates would not be seen for some time yet."

For Dr. Corneila Baines, professor emerita at the Dalla Lama School of Public Health at the University of Toronto, a crucial question remains unanswered: what is the risk for women who have had hysterectomies and are on estrogen only?

Last month, the Society of Obstetricians and Gyecologists of Canada issued new guidelines that said hormone replacement therapy is effective and safe for periods of up to five years to alleviate symptoms such as hot flashes.

Official guidelines recommend that women take hormone therapy for serious menopausal symptoms for the shortest time possible and at the lowest dose.