A new study has reported that perimenopausal and postmenopausal women treated with metformin for type 2 diabetes are 23% less likely to develop breast cancer than similar diabetic women not treated with metformin. Evidence is mounting that metformin may reduce risk of breast cancer by lowering insulin levels and reducing cell proliferation. The study included Danish 4,323 women aged 50 years or older with type 2 diabetes identified from national medical registries. The women resided in northern Denmark during 1989 to 2008 and included 393 breast cancer cases and 3,930 cancer-free controls. Ninety-six of the breast cancer cases and 1,154 of the controls used metformin for at least one year.

The women with breast cancer were on average slightly older than those without breast cancer, but they were similar in terms of number of children, use of hormone replacement therapy, and history of diabetic complications. Women treated with metformin were found to be 23% less likely to receive a diagnosis of breast cancer than nonmetformin users. These results held after adjusting for clinically diagnosed obesity, diabetes complications, and important predictors of breast cancer. The authors conclude that metformin may protect against breast cancer in type 2 diabetic perimenopausal and postmenopausal women. The authors comment that the study supports the growing evidence of a role for metformin in breast cancer chemoprevention.

Please see our article on type 2 diabetes and breast cancer for more information on the relationship between breast cancer and type 2 diabetes, including its treatment.