Oseltamivir belongs to the family of medicines called antivirals, which are used to treat infections caused by viruses. Oseltamivir is used in the treatment of the infection caused by the flu virus (influenza A and influenza B). Oseltamivir may also be used to prevent and treat swine influenza A. Oseltamivir may reduce flu symptoms (weakness, headache, fever, cough, runny or stuffy nose, and sore throat) by 1 day. Oseltamivir is also used to prevent influenza infection if you have come into close contact with someone who has the flu. If you receive the flu vaccine every year, continue to do so. Oseltamivir is not a substitute for your yearly flu shot. This medicine is available only with your doctor's prescription.