Metformin usage and the risk of colorectal cancer: a national cohort study

Purpose: This study aimed to investigate the association between metformin usage and the risk of colorectal cancer (CRC) using data from the Korean National Health Insurance Service–National Health Screening Cohort database. Methods: Data from the NHIS-HEALS cohort between 2002 and 2015 were longitudinally analyzed. Subjects were divided into three groups: metformin non-users with diabetes mellitus (DM), metformin users with DM, and no DM group. CRC was defined using the ICD-10 code (C18.0-C20.0) at the time of admission. Cox proportional hazard regression models were adopted after stepwise adjustment for confounders to investigate the association between metformin usage and colorectal cancer risk. Results: During the follow-up period, of the total 323,430 participants, 2341 (1.33%) of the 175,495 males and 1204 (0.81%) of the 147,935 females were newly diagnosed with CRC. The estimated cumulative incidence of CRC was significantly different among the three groups based on Kaplan-Meier’s survival curve (p values < 0.05 in both sexes). Compared with metformin non-users, hazard ratios (95% CIs) of metformin users and the no DM group were 0.66 (0.51–0.85) and 0.72 (0.61–0.85) in males and 0.59 (0.37–0.92) and 0.93 (0.66–1.29) in females, respectively, after being fully adjusted. Conclusions: Metformin users with diabetes appear to have a significantly lower risk of CRC compared with metformin non-users.