Molnupiravir is a prodrug with antiviral activity against SARS-CoV-2. It is metabolized to a cytidine nucleoside analog, NHC, which is distributed in cells, where the NHC is phosphorylated to form a pharmacologically active ribonucleoside triphosphate (NHC-TP). Incorporation of NHC-TP (as NHC-monophosphate [NHC-MP]) into SARS-CoV-2 RNA by the viral RNA polymerase (nsp12) results in the accumulation of errors in the viral genome, leading to inhibition of replication. The mechanism of action (known as viral error catastrophe or viral lethal mutagenesis) is supported by biochemical and cell culture data, studies of SARS-CoV-2 infection in animal models, and analysis of SARS-CoV-2 genome sequences in humans treated with molnupiravir.