Metformin and the risk of prostate cancer across racial/ethnic groups: a population-based cohort study

BACKGROUND: Men with diabetes may have a lower risk of prostate cancer than men without diabetes which may be altered by metformin use or race/ethnicity. METHODS: Using administrative databases, from 1994 to 2012, adult (age 50 years) men with diabetes were identified. Metformin exposure was defined as a time-dependent variable, stratified first into any use, and into tertiles of cumulative dose. Surname algorithms identified individuals as Chinese or non-Chinese. Multivariable Cox regression estimated the risk of prostate cancer. RESULTS: The cohort of 80 001 had a mean age of 64 years and median follow-up of 9 years. Chinese users of metformin aged 50-59, 60-69 and 70 had similar risks of prostate cancer as non-users. Non-Chinese users aged 50-59 (adjusted hazards ratio (aHR): 0.86, 0.74 to 1.00) had a decreased risk whereas men aged 60-69 and 70 did not. However, when metformin exposure was stratified into tertiles, there was no association in any strata except non-Chinese men aged 50-59 in the first (aHR: 0.68, 95% confidence interval (CI): 0.55, 0.84), second (aHR: 0.75, 95% CI: 0.61, 0.92) and third (aHR: 0.79, 95% CI: 0.64, 0.96) tertiles of metformin exposure and non-Chinese men aged 60-69 in the first (aHR: 0.81, 95% CI: 0.68, 0.95) tertiles of metformin exposure. CONCLUSIONS: There was no clear association between metformin and risk of prostate cancer in men with diabetes in either race/ethnicity. Our findings suggest a consistent relationship between metformin and prostate cancer across race/ethnicity.