Carbamazepine serum levels are markedly and very rapidly increased by isoniazid, and
toxicity can occur. Rifampicin (
rifampin) has been reported both to augment and negate this interaction. Limited evidence suggests that
carbamazepine may potentiate
isoniazid hepatotoxicity.
Carbamazepine toxicity can develop quickly (within 1 to 5 days) and also seems to disappear quickly if the
isoniazid is withdrawn. Concurrent use should not be undertaken unless the effects can be closely monitored and suitable downward dosage adjustments made (a
carbamazepine dose reduction to between one-half or one-third was effective in some patients).
Carbamazepine toxicity may present as nausea, vomiting, ataxia or drowsiness.