Favipiravir (originally known as T-705) is an orally administered novel anti-viral compound with a unique mechanism of action that is active against a wide range of RNA-based viruses in laboratory tests. Favipiravir has recently being approved in Japan under the brand name Avigan. Avigan is an experimental antiviral drug being developed by Toyama Chemical of Japan. It is a viral RNA polymerase inhibitor with a new mechanism of action, inhibiting viral gene replication within infected cells to prevent the propagation. Favipiravir is phosphoribosylated by cellular enzymes to its active form, favipiravir-ribofuranosyl-5′- triphosphate (RTP). Favipiravir is active against a broad range of influenza viruses, including A(H1N1)pdm09, A(H5N1) and the recently emerged A(H7N9) avian virus. It also inhibits influenza strains resistant to current antiviral drugs, and shows a synergistic effect in combination with oseltamivir, thereby expanding influenza treatment options. A Phase III clinical evaluation of favipiravir for influenza therapy has been completed in Japan and two Phase II studies have been completed in the United States. In addition to its anti-influenza activity, favipiravir blocks the replication of many other RNA viruses, including arenaviruses (Junin, Machupo and Pichinde); phleboviruses (Rift Valley fever, sandfly fever and Punta Toro); hantaviruses (Maporal, Dobrava, and Prospect Hill); flaviviruses (yellow fever and West Nile); enteroviruses (polio- and rhinoviruses); an alphavirus, Western equine encephalitis virus; a paramyxovirus, respiratory syncytial virus; and noroviruses.