Breast cancer risk increases with long-term use of hormone therapies, according to a study that analyzed the California Teachers Study.

The study, led by Tanmai Saxena, an MD/PhD student at the Keck School of Medicine at the University of Southern California, analyzed hormone replacement therapy use among 2,857 women for almost 10 years.

The results of the study underscored the need for personalized risk-benefit discussions before women begin hormone therapy. Specifically, researchers found that compared with women who had never used hormone therapy, women who used estrogen therapy for  more than 15 years had a 19% greater risk of developing breast cancer. Furthermore, women who used combined therapy with estrogen plus progestin for 15 years or longer had an 83% greater risk. Breast cancer risk was highest among women who used the combination regimen.

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Researchers pointed out that breast cancer risk seemed dependent on body mass index (BMI) with those having a BMI less than 30 experiencing an increased risk of breast cancer with combined hormone therapy. While the risk was strongest among women with a BMI less than 25, obese women had no further increase in risk associated with using combined hormone therapy.

The results of the study also revealed that the risk of breast cancer was confined to tumors that were positive for both estrogen and progestin receptors.

“These results add new evidence that risk does vary by other personal characteristics,” said Susan Hankinson, ScD, professor of medicine at Harvard Medical School. “However, for now the public health message remains essentially the same. There is an increased risk of breast cancer from hormone use, and further studies will address the question of how specific that risk is.”

The findings are published in Cancer Epidemiology Biomarkers & Prevention (2010 Aug 10. [Epub ahead of print]).