Moderate
Established
Description
Phenytoin induces CYP3A4-mediated metabolism of dexamethasone, significantly reducing dexamethasone plasma concentrations and risking loss of efficacy (e.g., in cerebral edema or cancer treatment).
Mécanisme
Phenytoin upregulates CYP3A4 and P-glycoprotein, markedly accelerating dexamethasone metabolism; dexamethasone AUC can fall by 50–80% during concurrent phenytoin therapy.
Signification clinique
Clinical series in neuro-oncology and transplant patients document loss of dexamethasone efficacy (e.g., return of cerebral edema symptoms) within days of initiating phenytoin.
Prise en charge
Double the dexamethasone dose when phenytoin is co-administered; monitor clinical response closely; consider levetiracetam as an alternative anticonvulsant to avoid this interaction.