Background: Metformin use has been associated with decreased cancer risk and mortality. However, the effects of metformin on the development of colorectal adenomas, the precursors of colorectal cancers, are not defined. Aims: This study aimed to evaluate the potential effect of metformin on the incidence of colorectal adenomas in diabetic patients with previous colorectal cancer. Methods: Among 488 consecutive diabetic patients who underwent colonoscopic surveillance after curative resection of colorectal cancer between 1998 and 2008, 240 patients were enrolled in this study and were compared in two groups: 114 patients taking metformin and 126 patients not taking metformin. Patient demographics, clinical characteristics, and colorectal adenoma incidence rate were analysed. Results: After a median follow-up of 58 months, a total of 33 patients (28.9%) exhibited adenomatous colorectal polyps among the 114 patients who used metformin, compared with 58 (46.0%) patients with colorectal adenomas among the 126 patients who did not use metformin (odds ratio = 0.48, 95% confidence interval = 0.280-0.816, P= 0.008). After adjustment for clinically relevant factors, metformin use was found to be associated with a decreased incidence of colorectal adenomas (odds ratio = 0.27, 95% confidence interval = 0.100-0.758, P= 0.012) in diabetic patients with previous colorectal cancer. Metformin use in diabetic patients with previous colorectal cancer is associated with a lower risk of colorectal adenoma.
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