Background: Several epidemiologic studies have reported strong inverse associations between metformin use and risk of colorectal cancer, although time-related biases, such as immortal time bias, may in part explain these findings. We reexamined this association using methods to minimize these biases.Methods: A cohort study was conducted among 47,351 members of Kaiser Permanente Northern California with diabetes and no history of cancer or metformin use. Follow-up for incident colorectal cancer occurred from January 1, 1997, until June 30, 2012. Cox regression was used to calculate HRs and 95% confidence intervals (CIs) for colorectal cancer risk associated with metformin use (ever use, total duration, recency of use, and cumulative dose).Results: No association was observed between ever use of metformin and colorectal cancer risk (HR, 0.90; 95% CI, 0.76-1.07) and there was no consistent pattern of decreasing risk with increasing total duration, dose, or recency of use. However, long-term use (≥5.0 years) appeared to be associated with reduced risk of colorectal cancer in the full population (HR, 0.78; 95% CI, 0.60-1.02), among current users (HR, 0.78; 95% CI, 0.59-1.04), and in men (HR, 0.65; 95% CI, 0.45-0.94) but not in women. Higher cumulative doses of metformin were associated with reduced risk. In initial users of sulfonylureas, switching to or adding metformin was also associated with decreased colorectal cancer risk.Conclusions: Our findings showed an inverse association between long-term use of metformin and colorectal cancer risk. Findings, especially the risk reduction among men, need to be confirmed in large, well-conducted studies.Impact: If our findings are confirmed, metformin may have a role in the chemoprevention of colorectal cancer. Cancer Epidemiol Biomarkers Prev; 27(5); 525-30. ©2018 AACRSee related commentary by Jackson and García-Albéniz, p. 520.