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.