Zanamivir is a potent and highly selective inhibitor of neuraminidase, an enzyme on the surface of influenza viruses. Viral neuraminidase facilitates the release of newly formed viral particles from infected cells and may facilitate viral entry through mucus to epithelial cell membranes, facilitating infection of other cells. Inhibition of this enzyme, both in vitro and in vivo, results in impaired replication of influenza A and B viruses, with activity against all known influenza A neuraminidase subtypes.