Recent guidelines governing anti-diabetic medications increasingly advocate metformin as first-line therapy in all patients with type 2 diabetes. However, metformin could be associated with increased risk of acute kidney injury (AKI), acute dialysis and lactate acidosis in marginal patients. In a retrospective nationwide cohort study, a total of 168 443 drug-naïve patients with type 2 diabetes ≥50 years, initiating treatment with either metformin or sulphonyl in Denmark between 2000 and 2012 were included in this study (70.7% initiated treatment with metformin); calculation of 1-year risk of acute dialysis was based on g-standardization of cause-specific Cox regression models for acute dialysis, end-stage renal disease and death. One-year risks of acute dialysis were 92.4 per 100 000 (95% CI, 67.1-121.3) and 142.7 per 100 000 (95% CI, 118.3-168.0) for sulphonylurea and metformin, respectively. The metformin-associated 1-year risk of acute dialysis was increased by 50.3 per 100 000 (95% CI, 7.9-88.6), corresponding to a risk ratio of 1.53 (95% CI, 1.06-2.23), and a number needed to harm of 1988, thus providing evidence of potential concerns pertaining to the increasing use of metformin.