Theoretically, Tamiflu can be taken as a preventative. For example, if everyone in your household is sick it could prevent you from getting sick. However, I doubt you would want to be on this drug long term (throughout the whole flu season). It is quite expensive and comes in packages of five pills. Plus, it can cause nausea and other side effects. You would be better off getting a flu shot for prevention.

If you do get the flu, Tamiflu should be started ASAP (within first 36 hours). Otherwise, it is worthless. It typically shortens the flu for 1-2 days but you will still be sick for several days