Association between metformin use and risk of prostate cancer and its grade.

BACKGROUND: Metformin is commonly prescribed to treat type 2 diabetes. Recent evidence suggests that it may possess antitumoral properties. The aim of this study was to test the association between metformin use and risk of prostate cancer and its grade among men with diabetes.
METHODS: Data were obtained from population-based health-care administrative databases in Ontario, Canada. This retrospective cohort study used a nested case-control approach to examine the relationship between metformin exposure and the risk of prostate cancer within a cohort of incident diabetic men aged 66 years or older. We conducted four case-control analyses, defining case subjects as 1) any prostate cancer, 2) high-grade, 3) low-grade, and 4) biopsy-diagnosed. In each analysis, case subjects were matched to five control subjects on age and cohort entry date. Metformin exposure was determined based on prescriptions before cancer diagnosis, and adjusted odds ratios (aOR) were estimated using conditional logistic regression. All statistical tests were two-sided.
RESULTS: Within our cohort of 119 315 men with diabetes, there were 5306 case subjects with prostate cancer and 26 530 matched control subjects. Within the cancer case subjects, 1104 had high- grade cancer, 1719 had low-grade cancer, and 3524 had biopsy-diagnosed cancer. There was no association between metformin use and risk of any prostate cancer (aOR = 1.03, 95% confidence interval [CI] = 0.96 to 1.1), high-grade cancer (aOR = 1.13, 95% CI = 0.96 to 1.32), low-grade cancer (aOR = 0.94, 95% CI = 0.82 to 1.06), or biopsy-diagnosed cancer (aOR = 0.98, 95% CI = 0.84 to 1.02).
CONCLUSIONS: This large study did not find an association between metformin use and risk of prostate cancer among older men with diabetes, regardless of cancer grade or method of diagnosis.