Associations between Diabetes, Metformin, and Second Breast Cancer Among Women with Prior Ductal Carcinoma In Situ

Abstract

Background: Women with a history of ductal carcinoma in situ (DCIS) face a higher risk of second breast cancer, yet little is known regarding the association of type 2 diabetes and metformin use and risk of second breast cancer among women with a history of DCIS. Methods: We conducted a population-based nested case-control study within a cohort of women diagnosed with DCIS between 1995 and 2013, including 497 cases of second breast cancer and 965 matched controls. Results: Compared to those with no diabetes, we found that type 2 diabetes and diabetes duration were not significantly associated with second breast cancer risk. Compared with never users, recent metformin use was significantly associated with reduced risk (OR = 0.36; 95% CI: 0.14–0.95), particularly with ≥36 months of use (OR = 0.24; 95% CI: 0.07–0.86). Conclusions: These findings suggest that recent and long-term metformin use is associated with a reduced risk of second breast cancer among women with a history of DCIS. Further studies with larger diabetic populations and more complete medication histories should be conducted to confirm these findings and inform prevention strategies.