Pharmacology: Pharmacokinetics: Metformin hydrochloride is slowly and incompletely absorbed from the gastrointestinal tract; the absolute bioavailability of a single 500 mg dose is reported to be about 50% to 60%, although this is reduced somewhat if taken with food. Once absorbed, protein binding in plasma is negligible; the drug is excreted unchanged in the urine. The plasma half-life is reported to range from 2 to 6 hours after oral dose. Metformin crosses the placenta and is distributed in breast milk in small amount.