| Summary: | Molnupiravir is a novel antiviral agent for coronavirus disease 2019 (COVID-19) treatment. Warfarin is an oral anticoagulation agent with difficult management due to drug interactions. Here, we describe a case of international normalized ratio (INR) prolongation in a patient who administrated warfarin with molnupiravir for COVID-19. An increased INR at 3.80, enough to discontinue warfarin, was observed on the fifth day of molnupiravir therapy, although the warfarin dose and INR were stable at 4 mg/day and approximately 2.0 before the molnupiravir initiation, respectively. Factors that affect the INR, such as severe COVID-19, cytokine, diet, liver dysfunction, and the concomitant use of medications other than molnupiravir, were unlikely in this patient. This case suggests that healthcare physicians should be aware of the possibility of drug interaction between molnupiravir and warfarin.
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