Study results presented at the annual American Society of Clinical Oncology (ASCO) Meeting indicate that diabetic postmenopausal women are less likely to develop hormone receptor positive (ER+/PR+) breast cancer if they are taking metformin as part of their treatment regiment compared to diabetic women who are being treated without metformin. Metformin is a drug used to treat type 2 diabetes. While results have not been entirely consistent, emerging evidence suggests use of metformin may reduce breast cancer risk. However metformin's influence on breast cancer risk according to hormone receptor status has not been studied.

The study included postmenopausal women in the 156,229-member Women’s Health Initiative (WHI) cohort. A total of 9,277 medication-treated cases of diabetes were identified in the WHI and followed for an average of 11.1 years. The women self-reported invasive breast cancer diagnoses, which were then verified by medical record review.

The authors compared the influence of diabetes treatment regimens incorporating metformin on breast cancer risk with diabetes treatment not including metformin. Known breast cancer risk factors such as body mass index (BMI) and hormone replacement therapy (HRT) were taken into account in the analysis. Women without diabetes were used as controls for comparison. Women with diabetes tended to be older and were more likely to be African American, obese and less physically active than the WHI cohort as a whole. Diabetic women not taking metformin had breast cancer risk comparable to those without diabetes. On the other hand, diabetic women taking metformin had 38% fewer hormone receptor positive (ER+/PR+) breast cancers. The authors conclude that metformin use was associated with lower ER+/PR+ breast cancer incidence in diabetic women, providing support for testing metformin in breast cancer prevention studies.

Please see our article on type 2 diabetes and breast cancer for more information on how metformin influences breast cancer risk and outcomes.