Cigarette smoking and diabetes mellitus (DM) increase risks of cardiovascular diseases and all-cause mortality in cancer survivors. An increased risk of DM incidence in cancer survivors has been observed and smoking is an important modifiable risk factor for DM development in the general population. Thus, we investigated the association between smoking behavior change after cancer diagnosis and DM incidence in cancer survivors.

This retrospective cohort study using the Korean National Health Insurance System database included patients with newly diagnosed cancers between 2010 and 2016. Smoking behavior changes after cancer diagnosis were categorized as never smoker, ex-smoker, quitter, smoking starter, and persistent smoker. The associations between smoking behavior changes and DM incidence in overall and specific cancers were investigated using Cox regression analyses.

Of the 263,940 cancer survivors, 67.6 % were never smokers, 16.0 % were ex-smokers, 9.3 % were quitters, 1.0 % were smoking starters, and 6.2 % were persistent smokers. During a mean follow-up of 4.0 years, 12,175 patients were diagnosed with DM after cancer development. Compared to the never smokers, the adjusted hazard ratios (95 % confidential interval) of DM incidence were 1.06 (1.00–1.13) for ex-smokers, 1.45 (1.35–1.54) for quitters, 1.46 (1.25–1.71) for starters, and 1.57 (1.45–1.69) for persistent smokers.

Compared with never smokers, cancer survivors who engaged smoking at any point before or after cancer diagnosis showed an increased risk of DM incidence after cancer diagnosis. Cancer survivors should be advised to quit smoking promptly and to maintain abstinence throughout cancer survivorship.