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