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

• 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

Treatment

• 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