Amantadine, and its derivative rimantadine, are synthetic antiviral agents that appear to act on an early step of viral replication after attachment of virus to cell receptors. The effect seems to lead to inhibition or delay of the uncoating process that precedes primary transcription. Amantadine may also interfere with the early stages of viral mRNA transcription. Amantadine at usual concentrations inhibits replication of different strains of influenza A virus, influenza C virus, Sendai virus, and pseudorabies virus. It is almost completely absorbed from the GI tract, and ~90% of a dose administered PO is excreted unchanged in the urine over several days (human data). The main clinical use has been to prevent infection with various strains of influenza A viruses. However, in people, it also has been found to produce some therapeutic benefit if taken within 48 hr after the onset of illness. Amantadine and its derivatives may be given by the PO, intranasal, SC, IP, or aerosol routes. It produces few adverse effects, most of which are related to the CNS; stimulation of the CNS is evident at very high doses.