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Phenytoin-Warfarin Interaction — NDEB AFK MCQ

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HardPharmacologyPhenytoin-Warfarin InteractionNDEB AFK

A patient on phenytoin for epilepsy also takes warfarin for atrial fibrillation. Which important drug interaction should the dentist be aware of?

Educational content. Not a substitute for clinical judgement or local policy.

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Correct answer: C — Phenytoin induces hepatic enzymes that increase warfarin metabolism, potentially reducing its anticoagulant effect

Phenytoin is a potent inducer of hepatic cytochrome P450 enzymes (CYP2C9, CYP3A4), which increases the metabolism and clearance of warfarin, potentially reducing its anticoagulant effect and placing the patient at risk of thromboembolic events. This interaction means the patient's INR may be lower than expected. The dentist should be aware that the INR may not accurately reflect the anticoagulant state if phenytoin has been recently started, stopped, or dose-adjusted. Medical consultation is advisable before dental surgery.

Reference: Katzung – Basic and Clinical Pharmacology, 15th ed., 2021