Oseltamivir  of infection with influenza viruses A (including pandemic H1N1) and B. Oseltamivir exerts its antiviral activity by inhibiting the activity of the viral neuraminidase enzyme found on the surface of the virus, which prevents budding from the host cell, viral replication, and infectivity.

The clinical benefit of use of oseltamivir is greatest when administered within 48 hours of the onset of influenza symptoms since effectiveness decreases significantly after that point in time; there is generally no benefit in use beyond 48 hours for healthy, low-risk individuals as influenza is a self-limiting illness. However, antiviral treatment might be beneficial when initiated after 48 hours for patients with severe, complicated or progressive illness or for hospitalized patients. According to the CDC, data from clinical trials and observational studies have demonstrated that early antiviral treatment can shorten the duration of fever and illness symptoms, and may reduce the risk of some complications (including pneumonia and respiratory failure).