Depression and the risk of developing dementia: Analysis of the National Health Insurance Service Senior Cohort 2002-2019

Background: Depression has been repeatedly associated with increased dementia risk, but uncertainties remain regarding the temporal sequence, confounding factors, and the robustness of effect estimates. Prior studies often relied on prevalent depression diagnoses, short wash-out periods, or limited covariate adjustment, leaving ambiguity about whether depression precedes or follows prodromal cognitive decline. To evaluate the association between newly diagnosed depression and the risk of incident dementia among older adults in South Korea, using a rigorous design that improves temporal clarity and reduces confounding. Methods: We conducted a 12-year nationwide cohort study using the National Health Insurance Service–Senior Cohort Data (NHIS-SCD). Adults aged 65 years or older without a history of depression or dementia during a 6-year wash-out period (2002–2007) were followed from 2008 to 2019. Newly diagnosed depression cases in 2008 were identified (n = 5,974) and propensity-score matched (1:5) to controls (n = 29,784) based on age, sex, Charlson Comorbidity Index, and major chronic conditions. Cox proportional-hazards models with robust variance estimators were used to estimate hazard ratios (HRs) for dementia, adjusting for residual imbalances. Subgroup analyses examined effects by depression severity, sex, and age. Results: Over the follow-up period, incident dementia occurred more frequently in the depression group compared to controls. After matching and adjustment, newly diagnosed depression was associated with a significantly higher hazard of dementia (HR=1.78, 95% CI: 1.64–1.92, p<.001). Conclusion This large, propensity-matched cohort study provides robust evidence that a first diagnosis of depression in later life significantly increases the risk of developing dementia. Future research should focus on reducing depression severity and its long-term impact on dementia risk.