Introduction: Initiating favipiravir in COVID-19 patients with long-term warfarin use can lead to increased INR. However, data on the onset and duration of the increasing INR are limited. Method: We reviewed patient charts to include COVID-19 adult patients who received favipiravir for at least 5 days and used warfarin at the same dose for at least 12 weeks. Data on demographics, comorbidities, other medical characteristics, international normalized ratio (INR), and signs of bleeding were collected. Result: Eight patients, with a mean age of 70.88 ± 8.49 years old, received the standard dose of favipiravir. The mean maximum INR (4.30 ± 1.26) was statistically different from the baseline INR (P = .00029) and the change was observed within 4.38 ± 1.99 days after initiating favipiravir. Warfarin was then discontinued without favipiravir discontinuation in most patients, allowing the INR to gradually decrease within 2 to 3 days. Conclusion: Concurrent use of favipiravir and warfarin led to INR prolongation within approximately 4 days. The effect of such interaction can be acute as the prolongation occurred within 1 day in 1 of the patients.
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