Among the medications, most JM6 researchers were interested in the most commonly used antidiabetic drug, metformin. Noto et al. reported a systemic review and meta-analyses that the use of metformin in diabetic patients was associated with significantly lower risks of HZ52 cancer mortality and incidence. Lai et al. reported anti-diabetic medications such as metformin, thiazolidinediones, and alpha-glucosidase inhibitors considerably decreased the risk of lung cancer. However, Smiechowski et al. reported metformin use is not associated with a decreased risk of lung cancer in an UK database. Finally, someone may consider that reduced life expectancy as a result of diabetes itself may decrease the incidence of lung cancer that occurs more frequently in later life. However, in our study, the mean following time of the diabetic cohort was just a little shorter than that of the non-diabetic cohort. The strengths of our study included its use of population-based data that are highly representative of the general population. However, certain limitations to our findings should be considered. First, the National Health Insurance Research Database does not contain detailed information regarding smoking habits, diet preference, occupational exposure, drug history, and family history, all of which may be risk factors for lung cancer. Second, the evidence derived from a retrospective cohort study is generally lower in statistical quality than that from randomized trials because of potential bias related to adjustments for confounding variables. Despite our meticulous study design and attempt to control for confounding factors, bias resulting from unknown confounders may have affected our results. Third, all data in the NHIRD are anonymous. Thus, the relevant clinical variables, such as serum laboratory data, pulmonary function tests, imaging results, and pathology findings were unavailable for the patients in our study. Otherwise, the data regarding COPD, T2DM, and lung cancer diagnoses were nonetheless reliable. Last, although treatment effect may be critical for evaluating the association from T2DM to lung cancer.