Valproic acid and ethosuximide interaction.

The addition of valproic acid to ethosuximide in treatment to prevent seizures caused increased serum concentration of ethosuximide in four of five patients observed. Ethosuximide levels increased from 73 to 112 microgram/ml (53% higher), with concomitant toxicity. Both were reversed by reduction of the ethosuximide dose from 27.4 to 20.4 mg/kg. Serum concentrations of these two drugs should be monitored closely when they are given together.