apixaban and phenytoin

Phenytoin is a potent inducer of CYP3A4 and P-glycoprotein (P-gp). Apixaban is a substrate for both CYP3A4 and P-gp. Induction of these pathways increases the metabolism and intestinal efflux of apixaban, leading to reduced systemic exposure and lower plasma concentrations of apixaban.

Reduced apixaban exposure may result in inadequate anticoagulation, increasing the risk of thrombotic events including stroke and venous thromboembolism. Patients with atrial fibrillation or those treated for VTE may have reduced protection against clotting.

Avoid co-prescription where possible. Consider alternative anticoagulants (e.g., warfarin with INR monitoring) or alternative antiepileptics (e.g., levetiracetam). If concurrent use is unavoidable, monitor for clinical signs of thrombosis and consider increasing the apixaban dose, though specific dosing recommendations are not well established.

Well-established interaction based on the known inducing properties of phenytoin and pharmacokinetic data for apixaban. Co-administration studies have demonstrated significant decreases in apixaban area under the curve.