Re: "Reduced Risk of Lung Cancer With Metformin Therapy in Diabetic Patients: A Systematic Review and Meta-Analysis"
Abstract
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 conclusion 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 regard 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. Suissa and Azoulay (9) recently outlined how time-related biases in observational studies distort the association between metformin and cancer risk. They explicitly addressed one of the studies (2) mentioned in the meta-analysis by Zhang et al. and noted that it suffered from a substantial time-lag bias. Zhang et al. appear unaware of the report by Suissa and Azoulay and its conclusions. In another study (3), investigators used a medical-claims database that included prevalent and incident metformin users and did not provide data on duration of diabetes or metformin use, drug switching, or time to cancer event. They calculated odds ratios that compared metformin users with…
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Evidence Hub reads this as a meta-analysis, done in people. Its evidence level is 1 of 5 (Highest · pooled evidence). Pooled evidence from several studies sits at the top of the evidence pyramid.
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