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.