The only apparent health problem in women taking estrogen alone is an increased risk of strokes and blood clots -- a risk doctors said might be reduced if women took lower doses of natural estrogen.

The new analysis of results from the government-sponsored Women's Health Initiative was published in the current issue of the Journal of the American Medical Association.

The paper confirmed what the WHI investigators first reported two years ago: Estrogen does not cause breast cancer. In fact, there were fewer cases of breast cancer among women who took estrogen, but researchers noted that the difference was not statistically significant.

'Protective Effect Against Breast Cancer'

But in the new study, when the researchers excluded women who stopped taking their pills, the benefit was significant: Women who took estrogen had a 33 percent lower risk of being diagnosed with breast cancer.

Dr. Wulf Utian, a reproductive endocrinologist at the Cleveland Clinic and director of the North American Menopause Society, called the results "quite remarkable. They actually show a protective effect against breast cancer."

The women in the trial all had undergone hysterectomy, so they didn't take progesterone. That hormone, or a synthetic variation of it (progestin), generally is prescribed to prevent uterine cancer in women taking estrogen.

Progestin the Culprit?

Another part of the Women's Health Initiative looked at the combination of synthetic estrogen and progestin called Prempro. In that trial, whose results were reported in 2002, the women assigned to take Prempro had slightly more cases of breast cancer than those who took dummy pills, as well as more heart attacks and strokes. Hormone use plummeted in response.

The most recent analysis of the data suggests that the culprit is the progestin. The only apparent health problem in women taking estrogen alone is an increased risk of strokes and blood clots -- a risk doctors said might be reduced if women took lower doses of natural estrogen.

In February, an analysis of results from the estrogen-alone study said the hormone is heart-healthy if women start taking it at or around menopause. The hormone was found to be harmful only for women who wait until after 60, when they probably have the beginnings of heart disease.