Phenytoin auto-induction? Case reports.

Auto-induction of phenytoin metabolism is considered by many to be insignificant. Two cases, in which elimination of phenytoin apparently increased with continuous use, are presented. The first patient showed an increase in elimination capacity (Vmax) from 511 mg/d to 613 mg/d over a period of 5 months. The second patient showed an increase in Vmax from 512 mg/d to 570 mg/d over a period of 1 year. In neither case was compliance in question. The recognition that therapeutic failure may be due to enzyme induction could obviate expensive diagnostic procedures as well as narrow down the therapeutic alternatives.