Our patient had no features of phenytoin toxicity despite a concentration of 31-3 mg/I. After starting tolbutamide the phenytoin concentration decreased to 27-4 mg/I. The total concentration was measured, not the free level. Neither the Committee on Safety of Medicines nor the manufacturer knows of any other cases of phenytoin toxicity produced by tolbutamide.
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Simmonds et al. (1988) studied this question.