Baloxavir: what it treats, how to take it, side effects
Last updated September 3, 2026.
Baloxavir is a flu antiviral - a polymerase acidic endonuclease inhibitor that stops the flu virus from spreading in the body. You may know it as Xofluza. One dose, one time - and the clock matters: it works when started within 2 days of symptoms.
What does it treat?
Influenza in people 5 years and older with symptoms no longer than 2 days - otherwise healthy or at high risk of flu complications - and prevention of flu in people 5 and older after close contact with someone who has it. It shortens how long symptoms last. It does not prevent or treat the bacterial infections that can follow flu.
How do you take it?
One dose, once - tablet or the suspension (swirl gently, never shake; measure with the oral syringe; do not mix with other liquids or soft food). With or without food - but not with dairy products like milk or yogurt, and not with calcium-fortified drinks: calcium binds it and kills the dose. Same caution for calcium, iron, magnesium, aluminum antacids, and laxatives with those minerals - space them away per your pharmacist. The timing rule is the drug: past the 2-day window, the benefit drops away - flu symptoms mean a call or visit today, not tomorrow.
Side effects to know
- Common: vomiting, diarrhea, and nausea - usually brief.
- Serious - call your doctor immediately or get emergency care: the allergic set - rash, hives, swelling of face, lips, tongue, or throat, and trouble breathing. Rare, but the one-dose format means any reaction is worth naming fast.
Who should not take it?
Flag pregnancy and severely weakened immunity - the evidence base there is thinner, and your doctor weighs it. High-risk patients - asthma, heart disease, diabetes, age 65-plus - are exactly who the 2-day window matters most for: flu symptoms plus a risk condition is a same-day call, never a wait-and-see. It does not replace the annual flu vaccine.
When is it an emergency?
Trouble breathing, chest pain, confusion, or a fever that breaks and roars back - the pattern of flu turning bacterial - is urgent care, drug or no drug. Face or throat swelling is a 911 call. For an overdose, call Poison Control at 1-800-222-1222. Pymander's escalation routing is built and tested specifically for this class of decision; see the safety architecture working paper.
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Common questions
Why the 2-day window?
Flu virus replication peaks early - an antiviral that stops viral spread helps most while the virus is still multiplying, which is the first 48 hours of symptoms. Past that, the damage is done and the benefit shrinks. The practical consequence: flu symptoms - sudden fever, aches, cough - especially with a risk condition, mean contacting your doctor today. Same-day access is part of what makes this drug work.
Why can't I take it with milk?
Calcium and other minerals bind baloxavir in the gut and block absorption - MedlinePlus says no dairy products or calcium-fortified drinks with the dose, and the same logic covers calcium, iron, magnesium supplements, aluminum antacids, and mineral-containing laxatives. Water is the vehicle. Your pharmacist owns the exact spacing for your supplements.
How is baloxavir different from Tamiflu?
Dose count and mechanism: oseltamivir (Tamiflu) is twice daily for five days, blocking viral release; baloxavir is one dose once, blocking viral replication further upstream. Both need the early window. One-and-done is the appeal - no five-day adherence to blow - and for households, the post-exposure prevention use covers ages 5 and up after a close contact.
Does it replace the flu shot?
No - the vaccine trains your immune system before exposure and is still the foundation; baloxavir is the treatment (or post-exposure backstop) for when flu gets through anyway. Antivirals fight one infection once; the vaccine lowers the odds and the severity of every exposure all season. Both, not instead.
My fever broke and came back worse - what now?
That is the classic bacterial-complication pattern - flu weakens the airways, and pneumonia or a sinus or ear infection moves in behind it. Baloxavir explicitly does not prevent or treat bacterial infections. Fever returning after improvement, new chest pain, breathing trouble, or confusion: same-day medical care, antibiotics if indicated. Do not write it off as "the flu dragging on."
Can my child take it?
Treatment: ages 5 and up, dosed by weight - the suspension exists for children who cannot take tablets (swirl gently, never shake, measured with the oral syringe). Prevention after a household exposure: also 5 and up. The same dairy and mineral rules apply to kids - chocolate milk with the dose is the classic family mistake. The pediatrician owns the weight-based dose.