What is Tamiflu? Tamiflu is an antiviral medication that blocks the actions of influenza virus types A and B in your body. Tamiflu 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. Tamiflu will not treat the common cold. Tamiflu may also be used for purposes not listed in this medication guide.