Millions of people with diabetes receive metformin treatment every day, this being consistent with widely implemented treatment guidelines. Clinicians are apprehensive about the possibility of lactic acidosis amongst metformin-treated patients with renal impairment. An examination of outcomes for a large cohort of people with diabetes and treated with metformin examined the incidence of hospitalisation with acidosis amongst these people. The study cohort was comprised of a community-based cohort of 75 413 patients with diabetes in one large private health system (n = 75,413) with time-dependent assessment of eGFR stage from January 2004 until January 2017, and results were replicated in 67 578 new metformin users and 14 439 new sulfonylurea users from 2010 to 2015, sourced from 350 private US health systems).The mean age of those followed was 60.4 (15.5) years, and there were 2335 hospitalizations with acidosis over a median follow-up of 5.7 years (interquartile range, 2.5-9.9 years). Metformin was no more likely to be associated with incident acidosis for the total cohort, or amongst those in sub-groups with an eGFR 45 to 59 mL/min/1.73 m2 or eGFR 30 to 44 mL/min/1.73 m2 (adjusted HR, 1.09; 95% CI, 0.83-1.44). HOWEVER, metformin use WAS associated with a strongly increased risk of acidosis at eGFR less than 30 mL/min/1.73 m2 (adjusted HR, 2.07; 95% CI, 1.33-3.22).  Metformin use was associated with acidosis only at eGFR less than 30 mL/min/1.73 m2 with resulting supporting the notion that use of metformin in patients with type 2 diabetes and significant renal impairment should only be undertaken with great caution. Read more in the original study, published in the Annals of Internal Medicine