Key Takeaways

• Xofluza (baloxavir marboxil) is an antiviral drug that works by blocking a key influenza virus enzyme involved in viral replication.
• It is the only FDA-approved flu medication that targets the virus using an endonuclease inhibitor, unlike neuraminidase inhibitors such as Tamiflu (oseltamivir).
• Xofluza is taken as a single dose, usually based on your age and body weight, and works best when taken within 48 hours of symptom onset.
• Certain supplements like calcium, magnesium, antacids, dairy products, and laxatives can interfere with absorption.
• Most people tolerate Xofluza well, but some may experience side effects like diarrhea, headache, or sinusitis (swelling in the sinuses). Serious adverse reactions are rare.

What Is Xofluza?

Xofluza, the brand name for baloxavir marboxil, is an FDA-approved prescription medication used to treat uncomplicated influenza in adults and adolescents 12 years of age and older. Created by Genentech, it’s also used for post-exposure prophylaxis to help prevent flu after close contact with someone infected.

Unlike older flu medications, Xofluza is given as a single dose, making it one of the most convenient treatments available during flu season.

Xofluza’s Mechanism of Action

To understand how Xofluza works, it helps to know how the flu virus replicates. Once the influenza virus enters your respiratory system, it takes over your cells and uses viral enzymes to multiply and spread. One of these enzymes is called polymerase acidic endonuclease.

Xofluza is an endonuclease inhibitor

Baloxavir marboxil targets the flu virus by blocking this endonuclease enzyme. This enzyme is essential for a process called “cap snatching,” where the virus steals pieces of your own genetic material to replicate.

By inhibiting this enzyme:

• The influenza virus cannot complete replication.
• Virus levels drop more quickly.
• Symptoms such as fever, fatigue, body aches, and cough may resolve sooner.

This mechanism is different from Tamiflu, which is a neuraminidase inhibitor. Tamiflu (oseltamivir) prevents new viruses from leaving infected cells, whereas Xofluza stops the virus earlier in its life cycle.

Works against influenza A and B

Xofluza acts on both influenza A and influenza B, giving it broad coverage similar to other antiviral medications.

Works faster inside the body

Because Xofluza directly inhibits viral replication early, viral load reduction can happen rapidly in the first 24 hours.

Why Timing Matters

Like other antivirals, Xofluza works best when taken as soon as possible after flu symptoms begin.

The Centers for Disease Control and Prevention (CDC) recommends treatment within 48 hours of symptom onset for maximum benefit, particularly in high-risk groups such as:

• People with chronic medical conditions.
• Older adults.
• Individuals with weakened immune systems.
• Pregnant individuals.
• Young adolescents with complications.

Even if more time has passed, a healthcare provider may still recommend treatment for certain patients who face a higher risk of complications like pneumonia or bacterial infections.

How Is Xofluza Taken?

Single-dose treatment

Unlike Tamiflu (which requires twice-daily dosing for 5 days), Xofluza is taken one time only. The dose of Xofluza depends on:

• Body weight.
• Age (must be 12 years of age or older unless otherwise instructed by your provider).

This one-and-done approach makes it easier for patients who struggle to take a full 5 days of medicine.

What Interferes With Xofluza?

Xofluza has important interactions with certain minerals and OTC products.

Avoid taking Xofluza with:

• Calcium
• Magnesium
• Antacids
• Laxatives containing metals
• Dairy products
• Supplements containing zinc, iron, or selenium

These products bind the medication and reduce its absorption, making Xofluza less effective. A healthcare professional can help you plan safe timing for supplements if needed.

How Effective Is Xofluza?

In clinical trials, Xofluza:

• Shortened the duration of flu symptoms compared with a placebo.
• Reduced viral load faster than oseltamivir in some studies.
• Relieved symptoms such as cough, fever, sore throat, and bronchitis-like irritation more quickly in certain groups.

It is most effective for uncomplicated influenza in otherwise healthy patients, though it may also be considered for post-exposure prophylaxis.

How Xofluza Compares to Tamiflu

Xofluza

• Endonuclease inhibitor
• Single-dose treatment
• Strong viral load reduction
• More susceptible to mineral-related drug interactions
• Can be taken by patients 12 years of age and older

Tamiflu (oseltamivir)

• Neuraminidase inhibitor
• Requires 5 days of dosing
• Fewer interactions with foods, dairy, and minerals
• Can be taken by patients 2 weeks of age and older

Both are antiviral medications recommended by healthcare providers depending on timing, symptom severity, and individual needs.

Side Effects of Xofluza

Most people tolerate Xofluza well. The most common side effects include:

• Diarrhea
• Headache
• Sinusitis
• Mild cough or upper respiratory irritation

These are often similar to symptoms of the flu itself.

Serious side effects (rare)

Seek medical help if you experience:

• A severe allergic reaction with symptoms such as hives, swelling of your face, tongue, or throat, and trouble breathing.
• Persistent or worsening fever.
• Symptoms suggesting a bacterial infection, such as worsening bronchitis or sinus issues.

Seek immediate medical attention or call your healthcare provider for medical advice if you experience any persistent or serious side effects.

Can You Take Xofluza With Other Flu Treatments?

Xofluza should not be combined with:

• Certain supplements
• Mineral-containing OTC remedies
• Antacids
• Dairy around the time of dosing

Combining antiviral medications like Tamiflu and Xofluza is not recommended unless specifically directed by a healthcare provider.

When to See a Doctor

You should contact your provider if:

• Your symptoms don’t improve.
• You develop severe coughing or bronchitis.
• Flu symptoms get better, then suddenly worsen.
• You have difficulty breathing or a consistent high fever.
• You belong to a high-risk group, and your symptoms are severe.

Remember that not every flu case needs antivirals, but treatment may reduce symptom duration and complications.

Bottom Line

Xofluza works by blocking the influenza virus’s ability to replicate, stopping the illness at an earlier stage than traditional antivirals. As an endonuclease inhibitor, it targets a viral polymerase enzyme essential for viral growth.

This fast-acting antiviral is convenient because it’s a single dose, but timing and avoiding mineral-based foods or supplements are important for it to be fully effective. If you’re unsure whether Xofluza is right for your situation, especially during flu season, speak with your healthcare provider.

FAQs

How quickly does Xofluza start working?
It begins targeting viral replication within around 4 hours after taking the single dose. However, it typically takes longer for you to feel a noticeable improvement in your symptoms.

Is Xofluza better than Tamiflu?
Not necessarily. Xofluza works differently and is more convenient, but effectiveness depends on timing, age, and medical conditions.

Can you take Xofluza with the flu vaccine?
Yes. The flu vaccine prevents infection; Xofluza treats an active flu infection. They serve different purposes.

Do supplements interfere with Xofluza?
Yes—avoid calcium, magnesium, iron, zinc, selenium, dairy products, and antacids around the time of dosing.

Sources

• Xofluza (baloxavir marboxil) Prescribing Information. Genentech. Accessed Dec.15, 2025.