Metformin therapy and risk of colorectal adenomas and colorectal cancer in type 2 diabetes mellitus patients: A systematic review and meta-analysis

Recent evidence indicates that metformin therapy may be associated with a decreased colorectal adenoma / colorectal cancer risk in type 2 diabetes patients.However, results are not consistent.We therefore performed a systematic review and meta-analysis to assess the association between metformin therapy and risk of colorectal adenomas / colorectal cancer in type 2 diabetes mellitus patients.We searched the literature published before Aug 31, 2016 in four databases: PubMed, Embase database, CNKI and VIP Library of Chinese Journal.Summary risk estimates (adjusted OR/ adjusted RR/ adjusted HR) with their 95% confidence interval (95% CI) were obtained using a random effects model.Twenty studies (including 12 cohort studies, 7 casecontrol studies and 1 randomized controlled trial study) were selected in terms of data of colorectal adenomas or colorectal cancer incidence.Metformin therapy was found to be associated with a decreased incidence of colorectal adenomas (unadjusted OR=0.80, 95% CI: 0.71-0.90,p=0.0002).When the adjusted data were analyzed, the summary estimate decreased to 25% reduction in colorectal adenomas risk (adjusted OR=0.75, 95% CI: 0.59-0.97,p=0.03).Besides, a significant reduction of colorectal cancer risk was also observed (unadjusted OR=0.73, 95% CI: 0.62-0.86,p=0.0002).And when the adjusted data were analyzed, colorectal cancer risk for metformin users was decreased with a reduction of 22%, compared with non-metformin users and other treatment users (adjusted OR=0.78, 95% CI: 0.70-0.87,p<0.00001).Our meta-analysis suggested that metformin therapy may be associated with a decreased risk of colorectal adenomas and colorectal cancer in type 2 diabetes mellitus patients.