Metformin is not metabolized [5] and is excreted unchanged in the urine, with a half-life of ~5 h [6]. The population mean for renal clearance (CLr) is 510±120 ml/min. Active tubular secretion in the kidney is the principal route of metformin elimination. The drug is widely distributed into body tissues including the intestine, liver, and kidney by organic cation transporters [6]. There is a large interindividual variability in metformin pharmacokinetics as measured by differences in trough steady-state metformin plasma concentration ranging from 54 to 4133 ng/ml [7].