Metformin and cancer risk and mortality: a systematic review and meta-analysis taking into account biases and confounders.

Previous meta-analyses have shown that the antidiabetic agent metformin is associated with reduced cancer incidence and mortality. However, this effect has not been consistently demonstrated in animal models and recent epidemiologic studies. We performed a meta-analysis with a focus on confounders and biases, including body mass index (BMI), study type, and time-related biases. We identified 71 articles published between January 1, 1966, and May 31, 2013, through Pubmed, ISI Web of Science (Science Citation Index Expanded), Embase, and the Cochrane library that were related to metformin and cancer incidence or mortality. Study characteristics and outcomes were abstracted for each study that met inclusion criteria. We included estimates from 47 independent studies and 65,540 cancer cases in patients with diabetes. Overall cancer incidence was reduced by 31% [summary relative risk (SRR), 0.69; 95% confidence interval (CI), 0.52-0.90], although between-study heterogeneity was considerable (I(2) = 88%). Cancer mortality was reduced by 34% (SRR, 0.66; 95% CI, 0.54-0.81; I(2) = 21%). BMI-adjusted studies and studies without time-related biases also showed significant reduction in cancer incidence (SRR, 0.82; 95% CI, 0.70-0.96 with I(2) = 76% and SRR, 0.90; 95% CI, 0.89-0.91 with I(2) = 56%, respectively), albeit with lesser magnitude (18% and 10% reduction, respectively). However, studies of cancer mortality and individual organ sites did not consistently show significant reductions across all types of analyses. Although these associations may not be causal, our results show that metformin may reduce cancer incidence and mortality in patients with diabetes However, the reduction seems to be of modest magnitude and not affecting all populations equally. Clinical trials are needed to determine if these observations apply to nondiabetic populations and to specific organ sites. ©2014 American Association for Cancer Research.