MONDAY, Jan. 31 (HealthDay News) -- Women who begin hormone therapy at menopause, or before, face a greater risk for breast cancer than those who start the treatment later, a new study has found.

Though a number of earlier studies had concluded that hormone therapy increases breast cancer risk, the researchers wanted to know whether the timing of the therapy influences that risk, something they said few studies had examined.

To do that, they analyzed data from 1.13 million participants in the Million Women Study in the United Kingdom, concluding that the length of time between menopause and starting hormone therapy has a "substantial" effect on breast cancer risk.

Compared with women who began hormone therapy before or soon after menopause, those who started hormone therapy five years or more after menopause had little or no increased risk, the study found. The difference in risk was evident regardless of the type of hormone therapy, how long women used it and whether women were normal weight, overweight or obese.

The findings, published online Jan. 28 in the Journal of the National Cancer Institute, support similar findings from the Women's Health Initiative study in the United States, two experts wrote in an accompanying editorial.

More information

The American Cancer Society has more about hormone therapy and breast cancer risk.