Within one to three hours after administration, half the dose of Tamiflu, or oseltamivir, is excreted from the body, notes Drugs.com. Oseltamivir is also converted to oseltamivir carboxylate, which remains in the body longer with a half-life of six to 10 hours. Tamiflu is primarily eliminated through the kidneys.
Tamiflu is prescribed to patients with influenza who are 2 weeks of age or older, according to Drugs.com. Patients are advised to begin taking Tamiflu within two days of the start of symptoms, or after close contact with a person who has influenza. This medication is generally taken twice daily for five days.