phenytoin and valproic acid

Valproic acid inhibits the hepatic metabolism of phenytoin, principally by inhibiting the cytochrome P450 enzyme CYP2C9. It also displaces phenytoin from plasma protein binding sites, increasing the free fraction of phenytoin. These effects can lead to an increased total and free phenytoin concentration, although the interaction is complex because changes in protein binding can alter total levels while free levels may rise more significantly.

The patient may develop signs of phenytoin toxicity, including nystagmus, ataxia, drowsiness, confusion, and slurred speech. The risk of toxicity is higher when phenytoin levels are already in the upper therapeutic range or when valproic acid is added. Conversely, stopping valproic acid can lead to reduced phenytoin levels and potential loss of seizure control.

Monitor total and, where available, free phenytoin concentrations, as well as valproic acid levels, when initiating, adjusting, or discontinuing either drug. Consider reducing the phenytoin dose when valproic acid is added, and increase it when valproic acid is withdrawn. Observe the patient for clinical signs of phenytoin toxicity and adjust the dose accordingly.

This interaction is well established in the literature and supported by multiple clinical studies and case reports. It is consistently documented in major drug interaction references and prescribing information.