Favipiravir, also known as T-705 or Avigan, is a pyrazine-derived antiviral used against RNA viruses. This molecule is an inhibitor of the viral RNA-dependent RNA polymerase. Favipiravir is believed to act selectively on RNA polymerases of viruses and not on those of host cells and it has no action on DNA viruses.  Favipiravir would therefore have very few side effects except in pregnant women because it would have a potential for teratogenicity and embryotoxicity in humans. This is why it is currently only marketed in Japan and banned in Europe.

Favipiravir has shown good results against influenza viruses, especially oseltamivir-resistant viruses. A first study, carried out in China, showed that favipiravir could reduce the healing time of Covid-19 from 11 to 4 days for mildly affected patients. Other studies are underway to determine whether favipiravir may be a good candidate for the treatment of Covid-19 caused by CoV-2 SARS.