Background: The risk of sudden sensorineural hearing loss associated with metformin use in patients with diabetes mellitus has not been fully examined. Study design: It is a retrospective matched-cohort study. Subjects and methods: We examined the medical records of patients with diabetes mellitus over 18 years old in Taiwan’s National Health Insurance Research Database for the period between 1 January 2000 and 31 December 2013, to establish matched cohorts (14,109 with and 42,327 without metformin use) at a ratio of 1:3 by sex, age and index year. Results: We used a Cox regression hazard model to identify risk factors of sudden sensorineural hearing loss during 14 years of follow-up, and the results indicate that a significantly lower percentage of diabetes mellitus patients with metformin use (p = 0.033) developed sudden sensorineural hearing loss compared with those without metformin use (0.21%, 29/14,109 vs 0.32%, 136/42,327). After adjustment for age and other variables adjusted hazard ratio: 0.630 (95% confidence interval: 0.422–0.941, p = 0.024), this study also demonstrated that metformin use appeared to reduce the risk of developing sudden sensorineural hearing loss. Conclusion: This study demonstrated an association between metformin use and lower incidence of sudden sensorineural hearing loss among patients with diabetes mellitus.