Metformin is associated with a lower risk of colorectal cancer in Taiwanese patients with type 2 diabetes: A retrospective cohort analysis

Abstract Background The association between metformin and colorectal cancer (CRC) has rarely been investigated in Asian populations. Methods This retrospective cohort study included patients with newly diagnosed type 2 diabetes during 1999–2005, recruited from Taiwan's National Health Insurance database. A total of 169,601 patients (original cohort: 153,270 ever-users and 16,331 never-users of metformin) and a subgroup of 1:1 propensity-score-matched pairs of 16,331 ever-users and 16,331 never-users (matched cohort) were followed up to 31 December 2011. Cox regression was constructed with the inverse probability of treatment-weighting, using propensity scores, and was used to estimate hazard ratios (HRs). Results In the original cohort, the incidence of CRC was 242.9 and 480.9 per 100,000 person-years, respectively, in ever- and never-users. The overall HR [0.50, 95% confidence interval (CI): 0.45–0.56] suggested a significantly lower risk in metformin users, while compared with never-users, the HR (95% CI) for the first (58.1 months) tertiles of cumulative duration of metformin therapy was 0.86 (0.76–0.98), 0.51 (0.45–0.59) and 0.26 (0.23–0.30), respectively. Analyses in the matched cohort showed similar findings with an overall HR of 0.62 (0.53–0.74), and a tertile analysis HR of 1.02 (0.81–1.28), 0.70 (0.56–0.89) and 0.32 (0.23–0.43), respectively. Re-analyses using more stringent diagnoses of CRC and cumulative duration as a continuous variable have consistently supported a protective effect with metformin use. Conclusion Metformin is associated with a lower frequency of CRC.