Ribavirin-13C5 (Standard) is the standard form of Ribavirin-13C5, suitable for quantitative analysis, quality control, and related research in biochemical experiments. Ribavirin-13C5 is intended to be used as an internal standard for the quantification of ribavirin by GC- or LC-MS. Ribavirin is an antiviral guanosine nucleoside analog. 1,2 Once it enters cells, ribavirin is metabolized into an active triphosphate form that causes viral RNA chain termination and inhibits viral polymerases. It reduces replication in a panel of seven RNA and four DNA viruses in Vero cells (EC50s = 2-95 μg/ml). 3 Ribavirin also reduces replication of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in Vero cells (EC50 = 109.5 μM). 4 Aerosol administration of ribavirin (30 mg/kg) reduces mortality in a mouse model of influenza A infection.5Formulations containing ribavirin have been used in the treatment of respiratory syncytial virus (RSV), hepatitis C virus (HCV), and viral hemorrhagic fevers.