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During the 1970s two biguanide drugs, phenformin and metformin, were used to control hyperglycemia. Phenformin was phased out of the Canadian market because it carried an unacceptable risk of causing lactic acidosis, but metformin remains available. All documented cases of lactic acidosis associated with metformin administration, which are rare, have occurred abroad in patients who were taking the drug in spite of having contraindications to its use. The two drugs are metabolized differently, phenformin being deactivated and concentrated in the liver, and metformin being excreted rapidly, unchanged, by the kidneys. In properly selected diabetic patients therapeutic doses of metformin do not raise the blood levels of intermediary metabolites enough to induce ketoacidosis or lactic acidosis. The safety of the drug is supported by the clinical experience over about 56,000 patient-years in Canada.