# Real-World Evaluation of the Interaction Between Rifampicin and Warfarin: A Retrospective Observational Study

## TL;DR

This study shows that combining rifampicin and warfarin significantly reduces warfarin's effectiveness, requiring dose adjustments and close monitoring.

## Contribution

The study provides real-world evidence of the clinical impact of the rifampicin-warfarin drug interaction through a large observational analysis.

## Key findings

- 86.3% of patients experienced a drug interaction, with a significant drop in INR levels.
- Warfarin doses were increased by a median of 5.5 mg to achieve target INR in 55.9% of patients.
- 31% of patients failed to reach target INR despite dose adjustments.

## Abstract

Background: Drug–drug interactions (DDIs) pose a significant challenge in drug therapy, particularly due to concerns about the safety and effectiveness of combined medications. Rifampicin is a strong inducer of the enzyme CYP2C9, which likely reduces warfarin's effectiveness. This study aims to investigate the prevalence and severity of clinically significant interactions by analyzing changes in international normalized ratio (INR) levels.

Method: The study was a retrospective observational analysis conducted from 2014 to 2024, using data on INR measurements from patients treated concurrently with warfarin and rifampicin. INR values were recorded at multiple time points, including baseline, during concomitant treatment, and after discontinuation.

Results: A significant proportion (86.3%) of the 102 patients using warfarin concurrently with rifampicin experienced a DDI, resulting in a notable reduction in INR (p < 0.0001), with a median decrease to 1.3 (IQR 1.1–1.6). Among the patients who achieved the target INR (55.9%), warfarin dose was increased by a median of 5.5 mg, and the median time to INR stabilization was 18 days. However, approximately 31% of patients did not reach the target INR despite dose adjustments.

Conclusion: The warfarin–rifampicin interaction is clinically significant, as it can diminish warfarin's anticoagulant effect, potentially compromising patient health outcomes. Close monitoring and individualized treatment plans are crucial for patients receiving both medications concurrently.