Dangerous Drug Interaction: Warfarin-Phenytoin

Impact: Potential for increased effects of warfarin and/or phenytoin¹

Mechanism of Interaction: Currently unknown, but one theory suggests a genetic basis involving liver metabolism of warfarin and phenytoin. Phenytoin may increase the effects of warfarin.⁶

Alternatives to Patient Management: Obtain baseline phenytoin levels prior to initiation of warfarin. Monitor INR during co-administration. Target INR should be towards the lower end of the therapeutic goal range.

Monitoring/Precautions: INR and phenytoin levels should be monitored during co-administration. Signs and symptoms of an active bleed should be monitored daily with particular attention to the appearance and patterns of bruises. Signs of an active bleed include: coughing up blood in the form of coffee grinds (hemoptysis), gingival bleeding, nose bleeds, cola- or tea-colored urine (hematuria), and black, tarry stools (hemoccult positive?).

References: Follow this link for a complete list of references.

The above information serves only as a guide for use by qualified medical practitioners in understanding, handling and avoiding frequent and potentially dangerous drug interactions that occur in long-term care. This presentation is not intended to instruct a practitioner how to treat any medical condition, nor is it intended to replace a practitioner's best clinical judgment. AMDA expressly disclaims responsibility and liability for any adverse effects, damages or other consequences resulting from the use of any of the information contained in this presentation.