The widely prescribed diabetes medicine metformin has been reported to lower the risk of incident breast cancer, but it is unclear whether it affects malignant progression after diagnosis. In this study, we conducted a retrospective cohort study using the linked Surveillance, Epidemiology, and End-Results (SEER)-Medicare database. Women were included in the study if they were aged 66 to 80 years, newly diagnosed with stage I or II breast cancer, and enrolled in Medicare Parts A, B, and D during 2007 to 2011. Information on dispensed diabetes-related medications was obtained from Medicare Part D claims data. Our primary outcomes were second breast cancer events (SBCE), breast cancer recurrence, and breast cancer death. Time-varying Cox proportional hazard models were used to estimate HRs and their 95% confidence intervals (CI). Among 14,766 women included in the study, 791 experienced SBCE, 627 had a recurrence, and 237 died from breast cancer. Use of metformin ( = 2,558) was associated with 28% (95% CI, 0.57-0.92), 31% (95% CI, 0.53-0.90), and 49% (95% CI, 0.33-0.78) lower risks of an SBCE, breast cancer recurrence, and breast cancer death. Use of sulfonylureas or insulin was associated with 1.49- (95% CI, 1.00-2.23) and 2.58-fold (95% CI, 1.72-3.90) higher risks of breast cancer death. Further research may be warranted to determine whether metformin is a preferred treatment for diabetes among breast cancer survivors and whether it benefits breast cancer patients without diabetes. .