Abstract: Acta Diabetologica (2021) 58:771–778 https://doi.org/10.1007/s00592-020-01666-7 ORIGINAL ARTICLE Metformin use and risk of COVID‑19 among patients with type II diabetes mellitus: an NHIS‑COVID‑19 database cohort study Tak Kyu Oh1 · In‑Ae Song1 Received: 7 September 2020 / Accepted: 26 December 2020 / Published online: 13 February 2021 © Springer-Verlag Italia S.r.l., part of Springer Nature 2021 Abstract Aims The relationship between metformin therapy and the risk of coronavirus disease (COVID-19) has not been reported among patients with type 2 diabetes mellitus (DM). We aimed to investigate whether metformin therapy was associated with the incidence of COVID-19 among type 2 DM patients in South Korea. Methods The National Health Insurance Service-COVID-19 cohort database, comprising COVID-19 patients from 1 January 2020 to 4 June 2020, was used for this study. Among them, adult patients with type 2 DM were included in this study. Metformin users were defined as those who had been prescribed continuous oral metformin for over a period of ≥ 90 days, and the control group was defined as all other patients. Results Overall, 27,493 patients with type 2 DM (7204, metformin user group; 20,289, control group) were included. After propensity score matching, 11,892 patients (5946 patients in each group) were included in the final analysis. In the logistic regression analysis, the odds of metformin users developing COVID-19 was 30% lower than that of the control group [odds ratio (OR): 0.70, 95% confidence interval (CI): 0.61–0.80; P < 0.001]. However, in the multivariate model, metformin use was not associated with hospital mortality when compared with that of the control group (OR: 1.26, 95% CI: 0.81–1.95; P = 0.301). Conclusions Metformin therapy might have potential benefits for the prevention of COVID-19 among patients with type 2 DM in South Korea. However, it did not affect the hospital mortality of type 2 DM patients diagnosed with COVID-19. Keywords Antidiabetic drug · Cohort study · Metformin · Type 2 diabetes