Metformin and lung cancer risk of patients with type 2 diabetes mellitus: A meta-analysis.

The association between metformin and the lung cancer risk of patients with type 2 diabetes mellitus (T2DM) remains controversial. Therefore, the present meta-analysis on epidemiological studies was performed to explore this issue. A comprehensive literature search was conducted for all the potential studies addressing metformin use and lung cancer risk by utilizing Pubmed, CBM and ISI Web of Science using the Mesh terms: 'Metformin,' or 'biguanides' and 'lung cancer,' or 'neoplasms'. The reference lists were also inspected. Eight observational studies, including 17,997 lung cancer patients, were eventually selected, which contained seven case-control and one cohort study. Compared to other antidiabetic agents, metformin was significantly associated with the 16% reduction of lung cancer risk in type 2 diabetic patients [relative risk (RR)=0.84; 95% confidence interval (95% CI), 0.73-0.97]. In the sensitivity analysis by separately excluding the study with a high weight or lower quality, the results did not materially change. Subsequently, subgroup analysis was performed on the type of study design, unadjusted or adjusted hazard ratio, quality of enrolled studies, duration of treatment, country and control drugs. The magnitude of lung cancer risk reduction was strengthened when compared to sulfonylureas (RR=0.79; 95% CI, 0.83-0.9), without significant heterogeneity (Q-value=2.98, P=0.085). In conclusion, the present analysis supported that the use of metformin significantly decreased the risk of lung cancer among patients with T2DM. However, further studies are required to confirm these findings.