Tamiflu is the trade name of the antiviral drug, Oseltamivir which has shown to slow the spread of influenza virus between cells in the body by stopping the new virus from chemically cutting ties with its host cell. Tamiflu is approved to treat the flu in adults and children over age 12 months. It is also indicated as prophylaxis, or a preventative, against the flu for adults and children over age 12 months. It is not a substitute for the flu shot; vaccination is the first line of defense for flu protection.