MAJOR — Use With Caution
FDA-documented bidirectional interaction. Monitor both INR and phenytoin levels when drugs are used together or when either is adjusted.
Evidence level: STRONG
Warfarin and phenytoin (Dilantin) have a complex two-way interaction. Phenytoin can make warfarin less effective, AND warfarin can increase phenytoin levels to toxic ranges. Both drugs compete for the same liver enzyme.
Complex bidirectional interaction: phenytoin induces CYP2C9 (increases warfarin metabolism) AND warfarin inhibits CYP2C9 (increases phenytoin levels). Both are CYP2C9 substrates. FDA labels document this.
Both your blood clotting (INR) and phenytoin levels need close monitoring. Changes to either drug dose will affect the other. Do not adjust either medication without your doctor knowing.
Take Warfarin at the same time each day (evening is common for warfarin). Phenytoin follows its prescribed schedule. Many medications can affect anticoagulant levels — always inform your pharmacist before starting anything new.
Starting/stopping/changing dose of either drug, concurrent illness
For warfarin: clotting events or bleeding depending on direction. For phenytoin toxicity: dizziness, double vision, slurred speech, unsteady walking, nausea.
Request monitoring of both INR and phenytoin levels. Discuss alternative antiepileptics like levetiracetam that have fewer interactions.
This medication combination requires careful medical oversight. Your doctor or anticoagulation clinic can adjust doses or select alternatives with fewer bleeding risks. Frequent INR monitoring is essential.
Warfarin and phenytoin (Dilantin) have a complex two-way interaction. Phenytoin can make warfarin less effective, AND warfarin can increase phenytoin levels to toxic ranges. Both drugs compete for the same liver enzyme.
Take Warfarin at the same time each day (evening is common for warfarin). Phenytoin follows its prescribed schedule. Many medications can affect anticoagulant levels — always inform your pharmacist before starting anything new.
For warfarin: clotting events or bleeding depending on direction. For phenytoin toxicity: dizziness, double vision, slurred speech, unsteady walking, nausea.
This medication combination requires careful medical oversight. Your doctor or anticoagulation clinic can adjust doses or select alternatives with fewer bleeding risks. Frequent INR monitoring is essential.
Request monitoring of both INR and phenytoin levels. Discuss alternative antiepileptics like levetiracetam that have fewer interactions.
Or browse the full interaction database (121,000+ pairs).