Phenytoin toxicity due to concomitant antituberculosis therapy.

Isoniazid inhibits the metabolism of phenytoin. Slow acetylators, who comprise roughly 50% of the South African population, are likely to develop clinical and biochemical features of phenytoin toxicity when this drug is given together with antituberculosis therapy. We describe a patient in whom this interaction caused a series of dangerous clinical events. Seventy-four per cent of patients with epileptogenic disorders seen at the Emergency Unit at Groote Schuur Hospital were on phenytoin and 11.6% of these had blood levels in the toxic range. The wide use of phenytoin during the recent tuberculosis epidemic makes it imperative to suspect this drug interaction in patients exhibiting clinical features that might be related to phenytoin toxicity. Knowledge of this interaction and adjustment of the dose of phenytoin should enable clinicians to avoid this adverse drug interaction.