Metformin, usually prescribed to help control blood glucose levels in persons with type 2 diabetes, may play a role in the prevention or management of breast cancer: The widely used oral drug has been linked with lower incidence of invasive breast cancer among participants in the Women’s Health Initiative (WHI) clinical trials.
Rowan T. Chlebowski, MD, PhD, of the Los Angeles Biomedical Research Institute at Harbor-UCLA Medical Center in Los Angeles, California, and colleagues explained in Journal of Clinical Oncology that although emerging evidence suggests metformin may reduce breast cancer incidence, reports are mixed and few provide information on tumor characteristics. To assess the associations among diabetes, metformin use, and breast cancer, the researchers studied WHI study participants.
Of the 68,019 postmenopausal women followed, 3,401 had diabetes at study entry. Over a mean of 11.8 years, 3,273 invasive breast cancers were diagnosed. Women with diabetes using medications other than metformin had a slightly higher incidence of breast cancer compared to women without diabetes, but women with diabetes who were metformin users were less likely to have breast cancer. The association was seen for cancers positive for both estrogen receptor (ER-positive) and progesterone receptor (PR-positive), and for those that were negative for human epidermal growth factor receptor 2 (HER2-negative).