For example, amantadine (Symmetrel) is a synthetic (man-made) drug that inhibits the multiplication of the influenza A virus and was once used to prevent the flu. Safer medicines such as oseltamivir (Tamiflu) and zanamivir (Relenza) can be used to prevent the flu shortly after exposure or to reduce the severity. Flu antiviral drugs work best for treatment when they are started within 2 days of getting sick. However, starting them later can still be helpful, especially if the sick person has a high risk health condition or is very sick from the flu. (Neither drug is a substitute for flu shots).