Smoking Cessation Duration and Diabetic Microvascular Complications.

Importance: The risk of diabetic microvascular complications is well established among smokers with type 2 diabetes. However, whether and to what extent this excess risk declines according to smoking cessation duration remains unclear. Objective: To investigate the association between smoking cessation duration and the risk of incident diabetic microvascular complications in patients with diabetes. Design, Setting, and Participants: This nationwide, population-based cohort study included participants first diagnosed with type 2 diabetes in Korea between 2003 and 2013 and whose smoking status was assessed at least 4 times across repeated health checkups, with a final checkup during 2009 to 2013. Data were analyzed from November 10, 2025, to April 16, 2026. Exposure: Smoking status and cessation duration. Quitters were defined as participants who reported current smoking at their initial health checkup and reported smoking cessation at all subsequent checkups without relapse. Continuous smokers were defined as participants who consistently reported current smoking at every health checkup. Among quitters, smoking cessation duration was categorized as less than 2, 2 to 3, 4 to 5, 6 to 7, and 8 or more years. Main Outcomes and Measures: Outcomes of interest were incident diabetic microvascular complications, including diabetic nephropathy, retinopathy, and neuropathy, identified using International Statistical Classification of Diseases and Related Health Problems, Tenth Revision (ICD-10) diagnostic codes. Cox proportional hazards models were used to estimate unadjusted and adjusted hazard ratios (aHRs) and 95% CIs. Results: A total of 209 698 adults (mean [SD] age, 57.2 [10.3] years; 127 823 [61.0%] female) were included in the analyses. Compared with continuous smokers, cessation for 8 or more years was associated with a lower risk of all microvascular complications (aHR, 0.81; 95% CI, 0.70-0.95), diabetic nephropathy (aHR, 0.79; 95% CI, 0.64-0.94), diabetic retinopathy (aHR, 0.59; 95% CI, 0.36-0.97), and diabetic neuropathy (aHR, 0.80; 95% CI, 0.66-0.98). Smoking cessation for 6 to 7 years was associated with lower risk of diabetic neuropathy (aHR, 0.76; 95% CI, 0.64-0.89). In subgroup analyses by sex, these associations were significant only among male participants (eg, all microvascular complications: aHR, 0.82; 95% CI, 0.70-0.95). Conclusions and Relevance: In this retrospective cohort study of patients with diabetes, smoking cessation for 8 or more years was associated with a lower risk of all diabetic microvascular complications, diabetic nephropathy, retinopathy, and neuropathy compared with continuous smoking. These associations were observed mainly among male participants, underscoring the potential importance of sustained long-term smoking cessation for microvascular risk reduction.