metformin is excreted unchanged by the kidneys69 70 and so it is renal function that determines metformin clearance. The principal contraindication to using metformin is renal impairment: the American Diabetes Association recommends avoiding metformin use if serum creatinine concentration exceeds 125 mmol/l.71 Because of the accumulation of lactate in hypoxia, metformin is also contraindicated in conditions such as uncontrolled heart failure that predispose to lactic acidosis.72