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For further information, see CMDT Part 38-38: Isoniazid Overdose

Key Features

  • Acute ingestion of as little as 1.5–2.0 g can cause toxicity

  • Severe poisoning is likely to occur after ingestion of more than 80–100 mg/kg

  • May cause hepatitis with long-term use

Clinical Findings

  • Confusion, slurred speech, and seizures may occur abruptly after acute overdose

  • Severe lactic acidosis—disproportional to the severity of seizures—is probably due to inhibited metabolism of lactate

  • Peripheral neuropathy and acute hepatitis may occur with long-term use


  • Diagnosis is based on a history of ingestion and the presence of severe acidosis associated with seizures

  • Isoniazid is not usually included in routine toxicology screening, and serum levels are not readily available


  • Administer activated charcoal after large recent ingestion but with caution because of the risk of abrupt onset of seizures

  • Seizures may require higher than usual doses of benzodiazepines (eg, lorazepam, 3–5 mg intravenously)

  • Pyridoxine (vitamin B6)

    • A specific antagonist of the acute toxic effects

    • Usually successful in controlling seizures that do not respond to benzodiazepines

    • Give 5 g over 1–2 minutes intravenously or if the amount ingested is known, give a gram-for-gram equivalent amount of pyridoxine

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