Background Although the mental health burden of diabetes has received substantial attention, there is limited evidence on whether the risk of suicide—a severe consequence of mental health problems—increases among individuals with diabetes. Therefore, this study investigated the association between diabetes and suicide in Japan.

Methods This nationwide longitudinal cohort study included adults aged 18–74 years, who were enrolled in the Japan Health Insurance Association’s health insurance programme between 2015 and 2022. Individuals newly diagnosed with diabetes were matched in a 1:1 ratio with those without diabetes, based on age and sex. We employed multivariable Cox proportional hazard models to estimate the hazard ratios for suicide in relation to the diagnosis of diabetes, adjusting for potential confounders, such as sociodemographic characteristics, history of psychiatric disorders, physical measurements, health behaviours and laboratory data.

Results Among 4 210 272 individuals in the matched-pair cohort, we observed 337 deaths by suicide among those with diabetes and 250 deaths by suicide among those without diabetes. After conditioning on potential confounders, diabetes diagnosis was linked to an increased risk of suicide (HR (95% CI) = 1.25 (1.06 to 1.47)). The association tended to be larger among those aged younger than 40 years and among women (among those aged 18–39 years, HR=1.69 (95% CI 1.05 to 2.73); among women, HR=1.56 (95% CI 0.92 to 2.64)).

Conclusion Diabetes diagnosis may increase suicide risk among the working-age population, even after conditioning on potential confounders. Our findings underscore the importance of psychosocial support following a diagnosis.