Oseltamivir is an antiviral medication that blocks the actions of influenza virus types A and B in your body. Oseltamivir is used to treat influenza in people 2 weeks of age and older who have had flu symptoms for 2 days or less. It may also be given to prevent influenza in people who are at least 1 year old, who may be exposed but do not yet have symptoms. Oseltamivir will not treat the common cold.