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.