Warfarin Dose Adjustment During Rifampicin Therapy: A Case Report with Practical Monitoring Implications

Warfarin management is challenging because its anticoagulant effect is influenced by drug-drug interactions and individual variability. Rifampicin is a potent inducer of cytochrome P450 enzymes and can substantially reduce warfarin efficacy, making structured monitoring and individualized dose adjustment necessary. This report aimed to describe the clinical management strategy for maintaining therapeutic anticoagulation in a patient receiving concurrent warfarin and rifampicin therapy, with a focus on International Normalized Ratio (INR) monitoring, dose adjustment, and pharmacist-led management. We present a clinical case of a 64-year-old Chinese male with chronic thromboembolic pulmonary hypertension who required long-term warfarin therapy and subsequently received rifampicin-containing antituberculosis treatment. The patient's clinical course, INR values, warfarin dose adjustments, and safety outcomes were retrospectively reviewed. A structured pharmacist-supported monitoring approach was applied during rifampicin co-administration. After rifampicin initiation, the patient's warfarin requirement increased from 1.5 mg/day to 4.5 mg/day, representing an approximately 200% increase. INR was monitored twice weekly during the initial phase and subsequently weekly until stabilization. A therapeutic INR range (2-3) was achieved after approximately 2 months without thromboembolic events or major bleeding complications. Rifampicin-warfarin co-administration requires individualized management rather than fixed dose adjustment strategies. Frequent INR monitoring, timely dose modification, and pharmacist involvement may facilitate safe maintenance of anticoagulation during this clinically significant drug-drug interaction.