Metformin and Gastrointestinal Cancer Development in Newly Diagnosed Type 2 Diabetes: A Population-Based Study in Korea.

INTRODUCTION Clinical studies have produced conflicting results on the effects of metformin on gastrointestinal cancer development. We aimed to investigate the association between metformin use and stomach, colon, liver, and pancreatic cancer development among patients with newly diagnosed, drug-naive type 2 diabetes. METHODS This retrospective study evaluated propensity score-matched patients with newly diagnosed type 2 diabetes from the Korean National Health Insurance Service database. Metformin users were categorized into tertiles according to the cumulative dose or duration of metformin treatment, and the risks of gastrointestinal cancers were compared. RESULTS Metformin users had reduced risks of developing stomach cancer (hazard ratio [HR]: 0.841, 95% confidence interval [CI]: 0.797-0.887), colon cancer (HR: 0.865, 95% CI: 0.822-0.91), and liver cancer (HR: 0.709, 95% CI: 0.675-0.746; P < 0.001). However, metformin users did not have a reduced overall risk of pancreatic cancer (HR: 1.335, 95% CI: 1.209-1.475; P < 0.001). The risks tended to decrease at higher cumulative doses and durations of metformin use, with significantly reduced risks of all 4 cancers at the highest cumulative dose (≥1,200,000 mg) and the longest duration (≥2,000 days) of metformin use. DISCUSSION This population-based data suggest that metformin could be associated with reductions in the risks of stomach, colon, and liver cancers, as well a reduced risk of pancreatic cancer in some subgroups. Metformin has benefit as a first-line treatment for type 2 diabetes mellitus. A further role in cancer risk reduction could be studied in controlled trials.