warfarin and phenytoin

Phenytoin is a potent inducer of hepatic CYP2C9 and other enzymes, increasing the metabolism of warfarin, which is a CYP2C9 substrate. Conversely, warfarin inhibits CYP2C9, reducing phenytoin metabolism. Both are narrow therapeutic index drugs. The interaction is complex: initial inhibition followed by induction, but clinically significant in both directions.

Patients may experience a decrease in anticoagulant effect, risking thromboembolic events such as stroke or deep vein thrombosis, due to induced warfarin metabolism. Conversely, warfarin-mediated CYP2C9 inhibition can raise phenytoin concentrations, leading to phenytoin toxicity (nystagmus, ataxia, drowsiness). The net effect depends on the timing and dose adjustments.

Avoid co-prescription if possible. If necessary, monitor INR frequently and adjust warfarin dose accordingly. Monitor serum phenytoin levels and signs of toxicity, adjusting phenytoin dose as needed. Consider alternative anticonvulsants or anticoagulants that do not interact (e.g., levetiracetam for seizures, or a DOAC if appropriate).

Well-established in clinical practice and pharmacokinetic studies. Both drugs interact via CYP2C9. Data from multiple case reports and controlled studies confirm the bidirectional interaction. Guidelines recommend close monitoring when these agents are combined.