Peramivir is a transition-state analogue and a potent, specific influenza viral neuraminidase inhibitor. Rapivab (peramivir injection) is indicated for the treatment of acute uncomplicated influenza in patients 18 years and older who have been symptomatic for no more than 2 days. Efficacy of Rapivab is based on clinical trials in which the predominant influenza virus type was influenza A and a limited number of subjects infected with influenza B virus were enrolled. Since influenza viruses change over time emergence of resistance substitutions could decrease drug effectiveness. Other factors such as changes in viral virulence might also diminish clinical benefit of antiviral drug. Efficacy could not be established in patients with serious influenza requiring hospitalization.