Letter to the Editor RE: “REDUCED RISK OF LUNG CANCER WITH METFORMIN THERAPY IN DIABETIC PATIENTS: A SYSTEMATIC

We read with great interest the meta-analysis by Zhang et al. (1). The authors summarized the most recent evidence available (2009–2013) to address the question of whether use of metformin is inversely associated with the risk of cancers of the lung and/or respiratory system. They came to the con-clusion that metformin use appears to be associated with lower risks of lung and respiratory cancers. We contest this conclusion. Zhang et al. included 6 observational studies, 4 of which were cohort studies and 2 of which were case-control studies (2–7). They mention that “each study was evaluated with re-gard to the [Strengthening the Reporting of Observational Studies in Epidemiology] statement [8] for ascertainment of data quality ” (1, p. 12). However, they appear to have missed (or ignored) the issue of bias