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 de velop clinical and biochemical features of phenytoin toxicity when thi s 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 disorder s seen at the Emergency Unit at Groote Schuur Hospital were on phenyto in and 11,6% of these had blood levels in the toxic range. The wide us e of phenytoin during the recent tuberculosis epidemic makes it impera tive to suspect this drug interaction in patients exhibiting clinical features that might be related to phenytoin toxicity. Knowledge of thi s interaction and adjustment of the dose of phenytoin should enable cl inicians to avoid this adverse drug interaction.