Tamiflu (oseltamivir) is taken as a 75 mg capsule twice daily for five days when treating the flu in adults and adolescents 13 and older. The medication works best when started within 48 hours of your first symptoms, and taking it with food helps reduce the nausea and vomiting that are its most common side effects.
Standard Adult Dosing
For flu treatment, adults take one 75 mg capsule every 12 hours for five days. You can take it with or without food, but eating something beforehand significantly lowers your chance of stomach upset. The full five-day course matters even if you start feeling better after two or three days. Stopping early doesn’t give the drug enough time to fully suppress the virus.
Tamiflu works by blocking a protein on the surface of the flu virus that allows newly made viral copies to break free from infected cells. Without that escape mechanism, the virus can’t spread efficiently through your respiratory tract. This is why timing matters so much: the drug doesn’t kill the virus outright, it contains it. Starting treatment early, when viral levels are still climbing, cuts illness duration by one to three days and reduces severity.
The 48-Hour Window
You’ll hear “start within 48 hours” repeatedly, and it’s the single most important factor in how well Tamiflu works. The clock starts when your symptoms begin, not when you test positive or see a doctor. If you wake up with body aches and fever on Monday morning, ideally you’d have your first dose by Wednesday morning at the latest.
That said, treatment beyond 48 hours isn’t automatically useless. The CDC recommends antiviral treatment as early as possible for anyone who is hospitalized or at higher risk for flu complications, regardless of how many hours have passed. For otherwise healthy adults with mild symptoms, the benefit after 48 hours drops considerably.
Dosing for Children
Children’s doses are based on body weight, and the treatment course is also five days. Kids who weigh more than about 88 pounds (40 kg) take the same 75 mg adult dose. For smaller children aged 1 to 12, the breakdown is:
- Under 33 lbs (15 kg): 30 mg twice daily
- 33 to 51 lbs (15 to 23 kg): 45 mg twice daily
- 51 to 88 lbs (23 to 40 kg): 60 mg twice daily
- Over 88 lbs (40 kg): 75 mg twice daily
Infants under one year receive weight-based liquid doses calculated by their doctor. Tamiflu comes as an oral suspension (liquid) for children who can’t swallow capsules, with a concentration of 12 mg per milliliter. If the pharmacy doesn’t have the liquid in stock, a pharmacist can compound it from the capsules.
Using Tamiflu for Prevention
If someone in your household has the flu and you want to prevent getting sick yourself, Tamiflu can be used as post-exposure prophylaxis. The prevention dose for adults is 75 mg once daily (instead of twice daily) for at least 10 days, started within 48 hours of close contact with the infected person. Children follow the same weight-based brackets listed above but take only one dose per day instead of two.
Prevention dosing is most commonly used for people at high risk of flu complications: older adults, people with chronic lung or heart disease, and those with weakened immune systems. It’s not a substitute for the flu vaccine, but it serves as a useful backup when you’ve had direct exposure.
What to Do About a Missed Dose
If you forget a dose, take it as soon as you remember. The one exception: if your next scheduled dose is less than two hours away, skip the missed one entirely and resume your normal schedule. Don’t double up to make up for it. Setting a phone alarm for roughly 12 hours apart (for example, 8 a.m. and 8 p.m.) helps keep you on track.
Side Effects to Expect
Nausea and vomiting are the most frequently reported side effects. In clinical trials involving children aged 1 to 12, vomiting occurred in about 15% of patients. Adults experience similar gastrointestinal discomfort, though taking the capsule with a meal or snack helps. These side effects are generally mild and tend to occur in the first couple of days of treatment before tapering off.
Headache is another commonly reported effect. Serious reactions are rare. If you notice any unusual behavioral changes, particularly in children or teenagers, contact your healthcare provider.
Storing the Liquid Form
Capsules can be stored at room temperature in a dry place. The liquid suspension has stricter requirements. Once prepared by the pharmacist, refrigerated liquid stays good for 17 days. If you can’t refrigerate it, it lasts 10 days at room temperature. Don’t freeze it. Your pharmacist will write the expiration date on the bottle, and any leftover suspension should be discarded after that date, even if there’s liquid remaining.