Background: Several in vitro studies have indicated that metformin may reduce the risk of prostate cancer; however, epidemiologic studies have been inconclusive. The objective of this study was to determine whether metformin decreases the risk of prostate cancer in patients with type 2 diabetes.

Methods: A nested case-control analysis was conducted within a population-based cohort from the UK General Practice Research Database. The cohort included patients over the age of 40 who were prescribed a first oral hypoglycemic agent (OHA) between 1988 and 2009. Cases of prostate cancer were matched up to ten controls on year of birth, date of cohort entry, and duration of follow-up. Adjusted rate ratios (RR) were estimated using conditional logistic regression.

Results: The cohort included 63,049 incident users of OHAs, in which 739 cases of prostate cancer were matched to 7,359 controls. Metformin use did not decrease the risk of prostate cancer (RR: 1.23, 95% CI: 0.99-1.52). In secondary analyses, prostate cancer risk was found to increase as a function of the number of metformin prescriptions received (one to seven prescriptions: RR: 1.05, 95% CI: 0.80-1.37; seven to eighteen prescriptions: RR: 1.29, 95% CI: 0.99-1.69; eighteen to thirty-six prescriptions: RR: 1.37, 95% CI: 1.04-1.81; more than thirty-six prescriptions: RR: 1.40, 95% CI: 1.03-1.89).

Conclusion: The results of this study indicate that metformin does not reduce the risk of prostate cancer in patients with type 2 diabetes.

Impact: The secondary analyses need to be interpreted with caution given the inverse association between type 2 diabetes and prostate cancer.