Metformin and other anti‐diabetic medication use and breast cancer incidence in the <scp>Nurses' Health Studies</scp>

Abstract We aimed to examine the association between the use of metformin and other anti‐diabetic medications and breast cancer incidence within two large prospective cohort studies. We followed 185,181 women who participated in the Nurses' Health Study (NHS; 1994–2016) and the NHSII (1995–2017), with baseline corresponding to the date metformin was approved for type 2 diabetes (T2D) treatment in the US Information on T2D diagnosis, anti‐diabetes medications, and other covariates was self‐reported at baseline and repeatedly assessed by follow‐up questionnaires every 2 years. Breast cancer cases were self‐reported and confirmed by medical record review. Hazard ratios (HRs) and 95% confidence intervals (CIs) for the association between medication use and breast cancer were estimated using Cox proportional hazards regression models, adjusting for breast cancer risk factors. During 3,324,881 person‐years of follow‐up, we ascertained 9,192 incident invasive breast cancer cases, of which 451 were among women with T2D. Compared with women without T2D ( n = 169,263), neither metformin use (HR = 0.97; 95% CI = 0.81–1.15) nor other anti‐diabetic medications use (HR = 1.11; 95% CI = 0.90–1.36) associated with significantly lower breast cancer incidence. Among women with T2D ( n = 15,918), compared with metformin never users, metformin ever use was not significantly inversely associated with breast cancer (HR = 0.92; 95% CI = 0.74–1.15). Although we observed that past use of metformin was inversely associated with breast cancer in the T2D population (HR = 0.67; 95% CI = 0.48–0.94), current use (HR = 1.01; 95% CI = 0.80–1.27) and longer duration of metformin use were not associated with breast cancer (each 2‐year interval: HR = 1.01; 95% CI = 0.95–1.07). Overall, metformin use was not associated with the risk of developing breast cancer among the overall cohort population or among women with T2D.