Massive metformin overdose can cause metabolic acidosis with hyperlactatemia. A 55‐year‐old woman presented 5 h after multidrug overdose, including 132 g extended‐release metformin. Continuous venovenous haemodiafiltration (CVVHDF) and noradrenaline were commenced due to metabolic acidosis (pH 7.0, lactate 17 mmol l–1) and shock. Despite 3 h of CVVHDF, her acidosis worsened (pH 6.83, lactate 24 mmol l–1). Intermittent haemodialysis (IHD) improved acidosis (pH 7.13, lactate 26 mmol l–1) but again worsened (pH 6.91, lactate 30 mmol l–1) with CVVHDF recommencement. IHD (12 h), CVVHDF (26 h) and vasopressor support for 7 days resulted in survival. Measured metformin concentrations were extremely high with a peak of 292 μg ml–1 at 8 h postingestion. IHD, but not CVVHDF in this case, was associated with improvement in metabolic acidosis and hyperlactataemia. Pharmacokinetic analysis of metformin concentrations found a reduced apparent oral clearance of 8.2 l h–1 and a half‐life of approximately 30 h. During IHD, the apparent oral clearance increased to 22.2 l h–1 with an approximate half‐life of 10 h. The impact of prolonged oral absorption from a pharmacobezoar and redistribution of metformin from peripheral sites (including erythrocytes) on the pharmacokinetic profile cannot be determined from the data available.