Safety of Breastfeeding During Favipiravir Therapy: A Case Study

Favipiravir is a known anti-influenza agent that has been used globally, especially during the coronavirus disease 2019 (COVID-19) pandemic. This drug has been important for treating influenza and other emerging infectious diseases, although it was ultimately proven to be ineffective for COVID-19. Currently, no studies have reported the human-milk transfer of favipiravir at doses of 1,600-3,600 mg/day in the treatment of diseases, including COVID-19. A 38-year-old mother gave birth at 38 gestational weeks because of severe respiratory distress caused by COVID-19. Considering her illness and the inadequate information available on the safety of favipiravir in breastfeeding, she chose formula feeding for her infant. However, she still provided human milk and blood samples for the analysis of drug concentrations. Five concentrations of favipiravir in the mother's milk and four in maternal blood were measured (3.0-80.9 and 3.5-78.4 μg/ml, respectively). Metabolites of favipiravir were not measured. The relative infant dose was 15.2%. The favipiravir concentration in human milk was as high as 80.9 μg/ml at 1.3 hours after the medication was given, when maternal blood levels are considered to be at their highest; however, favipiravir concentration was low in human milk immediately before medication administration (5.9-9.7 μg/ml). Breastfeeding is not contraindicated in mothers undergoing favipiravir therapy and would be safer immediately before medication. Avoiding feeding at the peak time (1.3 hours after medication) minimizes infant exposure.