Reduced Risk of Colorectal Cancer With Metformin Therapy in Patients With Type 2 Diabetes

OBJECTIVE Both in vitro and in vivo studies indicate that metformin inhibits cancer cell growth and reduces cancer risk. Recent epidemiological studies suggest that metformin therapy may reduce the risks of cancer and overall cancer mortality among patients with type 2 diabetes. However, data on its effect on colorectal cancer are limited and inconsistent. We therefore pooled data currently available to examine the association between metformin therapy and colorectal cancer among patients with type 2 diabetes. RESEARCH DESIGN AND METHODS The PubMed and SciVerse Scopus databases were searched to identify studies that examined the effect of metformin therapy on colorectal cancer among patients with type 2 diabetes. Summary effect estimates were derived using a random-effects meta-analysis model. RESULTS The analysis included five studies comprising 108,161 patients with type 2 diabetes. Metformin treatment was associated with a significantly lower risk of colorectal neoplasm (relative risk [RR] 0.63 [95% CI 0.50-0.79]; P < 0.001). After exclusion of one study that investigated colorectal adenoma, the remaining four studies comprised 107,961 diabetic patients and 589 incident colorectal cancer cases during follow-up. Metformin treatment was associated with a significantly lower risk of colorectal cancer (0.63 [0.47-0.84]; P = 0.002). There was no evidence for the presence of significant heterogeneity between the five studies (Q = 4.86, P = 0.30; I(2) = 18%). CONCLUSIONS From observational studies, metformin therapy appears to be associated with a significantly lower risk of colorectal cancer in patients with type 2 diabetes. Further investigation is warranted.