Valproic acid has variable effects on carbamazepine concentrations. Cholestyramine may lower valproic acid plasma concentrations by impairing oral absorption. Colestipol may lower valproic acid plasma concentrations by impairing oral absorption. Valproic acid may increase concentrations of lamotrigine by inhibiting its metabolism. Valproic acid increases concentrations of phenobarbital by inhibiting its metabolism.
Phenytoin decreases valproic acid plasma concentrations by increasing hepatic metabolism.
Valproic acid has variable effects on phenytoin concentrations. Valproic acid may increase concentrations of phenobarbital that is produced from primidone.