Zanamivir is the first of a new class of selective influenza virus neuraminidase inhibitors. It inhibits both influenza A and influenza B virus replication in vitro. In the ferret model of influenza, zanamivir reduced viral replication and diminished pyrexia associated with the infection. Repeated passage of influenza virus in the presence of zanamivir could produce resistance in vitro. However, there have been no changes in sensitivity to zanamivir in any influenza virus isolates from patients receiving zanamivir in clinical trials. In experimental infection in humans, in which the virus replicates only in the nasal passages, intranasal zanamivir (3.6 to 16 mg) prevented infection with influenza virus. A combination of inhaled (10 mg) and intranasal (6.4 mg) zanamivir for 14 days was effective in preventing influenza in a nursing home setting during an influenza A virus outbreak. Inhaled zanamivir (10 mg) with or without intranasal zanamivir (6.4 mg) reduced the time to alleviation of influenza symptoms compared with placebo in patients with confirmed infection.