Article Title

Diabetes Treatment and Risks of Adverse Breast Cancer Outcomes Among Elderly Breast Cancer Patients: A SEER-Medicare Analysis

Publication Date



cancer, pharmacy, Medicare


Background: Metformin, a first-line diabetes treatment, is hypothesized to lower the risk of incident breast cancer, but it is unclear whether metformin or other antidiabetic medications influence the likelihood of adverse breast cancer outcomes.

Methods: A retrospective cohort study was conducted using the linked Surveillance, Epidemiology and End-Results (SEER)-Medicare database. Diabetic and nondiabetic women were included if they were aged 66–80 years, newly diagnosed with stage I or II breast cancer, and enrolled in Medicare Parts A, B and D during 2007–2011. Information on filled diabetes-related prescription medications was obtained from Medicare Part D claims data. Our primary outcomes of interest were a second breast cancer event and breast cancer-specific mortality. Time-varying Cox proportional hazard models were used to estimate hazard ratios (HR) and their associated 95% confidence intervals (CI).

Results: Among 14,766 women included in the study, 791 were identified as having had a second breast cancer event, 627 had a recurrence and 237 died from breast cancer. Use of metformin after breast cancer (n = 2,558, 17.3%) was associated with 28% (HR: 0.72, 95% CI: 0.57–0.92), 31% (HR: 0.69, 95% CI: 0.53–0.90) and 49% (HR: 0.51, 95% CI: 0.33–0.78) lower risks of a second breast cancer event, breast cancer recurrence and breast cancer death, respectively, compared to metformin nonusers. Use of sulfonylureas (n = 1,701, 11.5%) was associated with 1.49 (95% CI: 1.00–2.23) higher risk of breast cancer death. A 2.58-fold higher risk (95% CI: 1.72–3.90) of breast cancer death also was seen among insulin users (n = 1,099, 7.4%). In assessing potential confounding by indication, similar patterns were observed in analyses restricted to pharmacologically treated diabetic patients.

Conclusion: We observed variation in the relationship between different diabetes medications and risk of adverse breast cancer outcomes, with metformin associated with reduced risks and sulfonylureas and insulin with increased risks. Pending confirmation of these results, metformin may be a preferred treatment for diabetes among breast cancer survivors, and further research examining its benefits among nondiabetic patients may be warranted.




June 20th, 2017


August 10th, 2017