Xofluza (baloxavir marboxil) works by blocking a specific enzyme the flu virus needs to copy itself, stopping the infection at its source with a single dose.

How Xofluza Stops the Flu Virus

To understand how Xofluza works, it helps to know a little about how the flu virus operates inside your body.

When an influenza virus infects one of your cells, it essentially hijacks your cell's machinery to make copies of itself. The virus needs to create copies of its genetic material (RNA) to spread. To do this, it relies on a specific enzyme called cap-dependent endonuclease — part of the virus's own replication system.

Think of it like a photocopier. The virus needs this enzyme to start the copying process. Xofluza works by jamming that photocopier. It blocks the endonuclease enzyme, which means the virus can't copy its genetic material. No copies, no new virus particles. The infection stalls.

This is fundamentally different from how older flu drugs work, which is why Xofluza is classified as a cap-dependent endonuclease inhibitor — it's the first and only medication in this class approved for flu treatment.

What Happens in Your Body After You Take Xofluza

Here's the step-by-step process:

1. You take one dose by mouth. The 80 mg dose is for patients weighing 80 kg (about 176 lbs) or more.
2. Your body converts the medication into its active form. Baloxavir marboxil is a "prodrug" — your body metabolizes it into the active compound, baloxavir, which does the actual work.
3. Baloxavir reaches infected cells and binds to the endonuclease enzyme on the virus's polymerase complex.
4. The virus can no longer replicate. Without the ability to copy its RNA, the virus can't produce new particles to infect more cells.
5. Your immune system clears the remaining virus. With replication halted, your body's natural defenses can mop up the existing virus particles.

The result: the flu runs a shorter, milder course than it would without treatment.

How Long Does Xofluza Take to Work?

Clinical trials showed that Xofluza significantly reduced the duration of flu symptoms compared to placebo. Patients who took Xofluza experienced relief of symptoms roughly one day sooner than those who didn't receive treatment.

At the cellular level, Xofluza starts working quickly. Studies showed a significant reduction in viral load (the amount of virus in your body) within 24 hours of taking the dose — faster than Tamiflu in head-to-head comparisons.

However, it's important to set realistic expectations: Xofluza doesn't make the flu disappear overnight. You'll still feel sick for a few days. What it does is shorten the duration and potentially reduce the severity of your symptoms.

How Long Does Xofluza Last in Your Body?

One of the remarkable things about Xofluza is its long half-life. The active compound, baloxavir, has a half-life of approximately 79 hours. This means a single dose stays active in your body for several days — which is why one dose is sufficient, compared to Tamiflu's five-day, twice-daily regimen.

You don't need to take any additional doses. One pill and you're done.

What Makes Xofluza Different from Other Flu Medications?

There are several antiviral medications used to treat the flu. Here's how Xofluza compares:

Xofluza vs. Tamiflu (Oseltamivir)

• Mechanism: Tamiflu is a neuraminidase inhibitor — it blocks the virus from spreading out of infected cells to infect new ones. Xofluza blocks the virus from replicating in the first place. Think of it this way: Tamiflu locks the exits, while Xofluza shuts down the factory.
• Dosing: Tamiflu requires twice-daily dosing for five days (10 pills total). Xofluza is one dose.
• Speed of viral reduction: Xofluza reduced viral load faster than Tamiflu in clinical trials.
• Availability: Tamiflu is available as a generic, making it cheaper and more widely stocked. Xofluza remains brand-only as of 2026.

Xofluza vs. Zanamivir (Relenza)

• Relenza is an inhaled neuraminidase inhibitor, taken twice daily for five days. Xofluza is a single oral dose — simpler and easier for most patients.

Xofluza vs. Peramivir (Rapivab)

• Rapivab is a single-dose treatment like Xofluza, but it's given as an IV infusion in a healthcare setting. Xofluza is taken by mouth at home.

For a detailed comparison of all flu treatment options, see our guide to Xofluza alternatives.

Does Xofluza Work Against All Flu Strains?

Xofluza is effective against both influenza A and influenza B viruses. In non-clinical studies, it has also shown activity against certain avian influenza strains (like H5N1 and H7N9) and against oseltamivir-resistant strains, though these are not FDA-approved indications.

One consideration: in clinical trials, some patients developed viruses with reduced susceptibility to Xofluza (known as PA/I38T variant). This was more common in children and typically did not affect clinical outcomes in otherwise healthy patients, but it's something researchers continue to monitor.

Final Thoughts

Xofluza 80 Mg Dose Pack works by targeting the flu virus's replication machinery — specifically, the cap-dependent endonuclease enzyme that the virus needs to copy itself. This first-in-class mechanism allows it to work with just a single dose, reducing viral load faster than older neuraminidase inhibitors like Tamiflu.

If you'd like to learn more about Xofluza, check out our guides on uses and dosage, side effects, and drug interactions. Ready to fill your prescription? Use Medfinder to find a pharmacy with Xofluza in stock near you.