Drug Interactions: Fluconazole and Warfarin

Introduction to the Interaction

• Fluconazole inhibits the CYP2C9 enzyme, which metabolizes the more potent S-enantiomer of warfarin, leading to decreased warfarin clearance and increased plasma concentrations, according to the American Heart Association 1

Monitoring and Dose Adjustment

• The American Heart Association recommends monitoring INR closely within the first week of co-administration of fluconazole and warfarin, with initial check at day 2-3 and follow-up check at day 5-7 when interaction peaks 1
• Consider preemptive warfarin dose reduction of 25-33% when initiating fluconazole, as suggested by the American Heart Association, similar to recommendations for other strong CYP2C9 inhibitors 1

Patient-Specific Factors

• Patients with decreased renal function may experience more pronounced interactions due to reduced fluconazole clearance, and genetic factors such as CYP2C9 polymorphisms may intensify this interaction, as noted by the American Heart Association 1

Clinical Significance

• The interaction between fluconazole and warfarin is clinically significant and requires proactive management to prevent potentially serious bleeding complications while maintaining therapeutic anticoagulation, as emphasized by the American Heart Association 1

Other Considerations

• Other medications may further compound anticoagulant effects, and even short courses of fluconazole can significantly affect INR, highlighting the need for careful management and monitoring, as recommended by the American Heart Association 1