1,4-Pyrazine-3-carboxamide-based antiviral compounds have been under intensive study for the last 20 years. One of these compounds, favipiravir (6-fluoro-3-hydroxypyrazine-2-carboxamide, T-705), is approved for use against the influenza infection in a number of countries. Now, favipiravir is being actively used against COVID-19. This review describes the metabolism of favipiravir, the mechanism of its antiviral activity, clinical findings, toxic properties, and the chemical synthesis routes for its production. We provide data on the synthesis and antiviral activity of structural analogs of favipiravir, including nucleosides and nucleotides based on them.