Tamiflu® is an antiviral prescription medicine that is sometimes prescribed for influenza. Tamiflu® can lessen symptoms and make flu illness one to two days shorter, says the CDC. It can also help prevent flu in people who have come into close contact with someone who is sick with the flu. Some studies have even reported that it can decrease the duration of illness by 30% to 40%, and decrease flu severity by 40%, but only if it is taken in the first 36 to 48 hours of illness. The prescription medication can be taken by adults and children older than one year of age, during the first two days of onset of symptoms. It works by stopping the spread of the flu virus in the body.