phenytoin and fluoxetine

Fluoxetine inhibits the cytochrome P450 enzymes CYP2C9 and CYP2C19, which are responsible for the metabolism of phenytoin. This inhibition reduces phenytoin clearance, leading to elevated plasma concentrations. Phenytoin has a narrow therapeutic index, so even modest increases in its levels can produce toxicity.

Elevated phenytoin levels can cause dose-related toxicity, including nystagmus, ataxia, confusion, slurred speech, and potentially seizures. Conversely, discontinuing fluoxetine can lead to a fall in phenytoin levels, which may reduce seizure control. The interaction is clinically significant, especially when phenytoin levels are near the upper therapeutic range.

If concurrent use is required, monitor plasma phenytoin concentrations before and after initiating or discontinuing fluoxetine. Repeat levels within 1 to 2 weeks of any change, and adjust the phenytoin dose accordingly. Monitor the patient for clinical signs of phenytoin toxicity. Advise the patient to report any new neurological symptoms.

Well established through clinical reports and pharmacokinetic studies; listed in standard interaction references. The interaction is predictable and can be managed with monitoring.