A. Tamiflu (oseltamivir phosphate) is an antiviral drug. It works by attacking the flu virus to keep it from multiplying in your body and by reducing the symptoms of the flu. Tamiflu can sometimes keep you from getting the flu if you take it before you get sick. The term flu refers to illness caused by the influenza virus. The flu is a respiratory infection that can cause symptoms such as fever, chills, aches and pains, cough, and sore throat. The flu can range from mild common cold symptoms, to the typical flu symptoms described above, to life-threatening pneumonia and other complications, including secondary bacterial infections.