(HealthDay News) — For postmenopausal women with diabetes, use of metformin is associated with lower incidence of invasive breast cancer, according to a study published online June 11 in the Journal of Clinical Oncology.

Rowan T. Chlebowski, M.D., Ph.D., from Harbor-University of California at Los Angeles Medical Center in Torrance, and colleagues observed 68,019 postmenopausal women over a mean of 11.8 years, including 3,401 with diabetes at study entry. Medical information was collected at baseline and years one, three, six, and nine.

During 11.8 years of follow-up, 3,273 invasive breast cancers were diagnosed. The researchers found that breast cancer incidence in women with diabetes differed significantly by diabetes medication type, compared with incidence in women without diabetes. A slightly higher incidence of breast cancer was seen for women with diabetes taking medications other than metformin (hazard ratio [HR], 1.16; 95 percent confidence interval [CI], 0.93 to 1.45), while the incidence was lower for women with diabetes taking metformin (HR, 0.75; 95 percent CI, 0.57 to 0.99). The correlation was seen for tumors that were positive for both estrogen receptor and progesterone receptor as well as those that were negative for human epidermal growth factor receptor 2.

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“Metformin use in postmenopausal women with diabetes was associated with lower incidence of invasive breast cancer,” the authors write. “These results can inform future studies evaluating metformin use in breast cancer management and prevention.”

Several authors disclosed financial ties to the pharmaceutical industry.

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