Tamiflu (oseltamivir) starts acting against the flu virus after the first dose, but people usually notice the benefit over the next couple of days, especially if it is started early in the illness.

Onset of action

• The medication begins inhibiting the flu virus within a few hours of taking a dose, limiting how quickly the virus can spread in the body.

• Even though it starts working quickly at a molecular level, you may not feel much different right away because your immune system still has to clear the infection.

When you feel better

• In most studies, Tamiflu shortens the duration of flu symptoms by about 1–2 days if started within 48 hours of symptom onset; in some older or high‑risk adults, the benefit can be up to about 2–3 days.

• Many people begin to feel some improvement within 1–2 days of starting it, but you can still feel sick for several days, since untreated flu often lasts 3–7 days.

Importance of early use

• Tamiflu works best when started as soon as possible, ideally within 48 hours after symptoms begin; starting later generally makes it less effective at shortening the illness.

• For people at higher risk of complications (such as older adults, pregnant people, or those with chronic conditions), clinicians may still prescribe it even if more than 48 hours have passed, because it can still help in some severe cases.