Patients with type 2 diabetes (T2D) are at greater risk of suicide due to stresses associated with disease management. Smoking adds to this burden. However, its association with suicide mortality in this population remains unclear. This study aimed to evaluate the relationship between smoking and suicide mortality in this high-risk group.

This cohort study utilized the Korean National Health Insurance Service (NHIS) database. A total of 2,524,769 participants aged ≥ 20 years with T2D who underwent national health examinations in 2015–2016 were included. Participants were followed until suicide death or the end of the study period.

Over 5.8 years mean follow-up, 5,578 individuals died by suicide. Risk of suicide mortality was significantly higher—adjusted hazard ratio [95 % confidence interval]—in current smokers compared to never-smokers: 1.55 [1.44;1.67]. A dose–response pattern was observed among current smokers: 1.41 [1.22;1.63] for light smokers (<10 cigarettes/day) and 1.66 [1.52;1.81] for heavy smokers (≥20/day). In ex-smokers, only those with a smoking history of ≥ 30 years exhibited an elevated risk: 1.12 [1.02;1.24]. The association was particularly strong in women: 2.97 [2.39;3.69]; and individuals aged <65 years: 1.76 [1.61;1.92].

Current smoking was significantly associated with increased suicide mortality in patients with T2D, particularly among heavy smokers, women and those younger than 65 years. Long-term smoking history was also associated with elevated risk among ex-smokers.