As of October 2020, there is still no specific drug to treat COVID-19 as it 
rages worldwide. Favipiravir, indicated for the treatment of new and re-emerging 
influenza infections, has been suggested to be effective against SARS-CoV-2, 
although this is not yet fully validated. We administered favipiravir to a 
64-year-old female patient with COVID-19. Her symptoms resolved quickly after 
the start of treatment, with reduction of SARS-CoV-2 viral load, but she 
developed a fever again on day 12. Since the fever was relieved by 
discontinuation of favipiravir, and based on positive results with a 
drug-induced lymphocyte stimulation test, we diagnosed her with 
favipiravir-induced drug fever. A decrease in the serum concentration of 
favipiravir was observed along with resolution of the fever. The present case 
suggests that drug fever should be considered in the differential diagnosis of 
relapsing fever episodes in COVID-19 patients receiving favipiravir.