It is an open-ring analog of guanosine nucleoside (Figure 3) that was approved by the U.S. FDA in 1985 for the treatment of respiratory syncytial virus [48]. It is also used systemically for chronic hepatitis C virus (HCV) infection [49] and viral hemorrhagic fever [50]. The drug possesses broad-spectrum antiviral activity against both RNA and DNA viruses. To exert its antiviral activity, the drug is to be activated by phosphorylation to generate the triphosphate nucleotide that acts as an inhibitor of RNA synthesis and viral mRNA capping [51]. Other mechanisms have also been proposed to account for its broad spectrum of antiviral activity. Inhibition of host inosine monophosphate dehydrogenase by ribavirin-monophosphate and the resulting depletion of guanosine triphosphate (GTP) pool has been put forward to be another mechanism of action. Decreased intracellular GTP pool decreases viral protein synthesis and limits the replication of viral genome. Ribavirin is also a mutagen that leads to defective virions [52] and it has immunomodulatory actions [53]. Yet, the drug has a U.S. boxed warning pertaining to the risk of hemolytic anemia and potential complications during pregnancy [54].