Symptoms are usually seen within 45 min of acute overdose but may be delayed up to 2 h, when peak absorption occurs. Seizures may occur abruptly and are often generalized and tonic-clonic, but focal seizures have been described. Isoniazid rarely causes mania, depression, obsessive-compulsive disorder and psychosis, believed to result from its ability to act as an MAO inhibitor or through depletion of pyridoxine. Signs and symptoms include fever, lymphadenopathy and skin rashes. Laboratory studies are generally not helpful in making the diagnosis of acute INH toxicity but may identify complications.
Prehospital therapy is limited to supportive care and management of complications.
Treatment of patients with evidence of toxicity involves managing immediate life-threats, administering pyridoxine and supportive care. Gastric lavage should be considered once the airway is secured, followed by administration of activated charcoal at a dose 10 times that of the amount of INH ingested, or 50 g if the dose ingested is unknown. Lavage and activated charcoal may not be effective if administered more than 1-2 hours after an acute ingestion. Control of seizures will generally correct metabolic acidosis. Administration of sodium bicarbonate may be beneficial in severe cases.