Xofluza: Uses, Dosage & Side Effects

What Is Xofluza and What Is It Used For?

Xofluza contains the active substance baloxavir marboxil, a prodrug that is rapidly converted in the body to its active form, baloxavir acid (commonly referred to simply as baloxavir). Baloxavir represents the first member of an entirely new class of antiviral drugs known as cap-dependent endonuclease (CEN) inhibitors. This mechanism of action is fundamentally different from all previously available influenza antivirals, including the neuraminidase inhibitors oseltamivir (Tamiflu) and zanamivir (Relenza), as well as the older M2 ion channel blockers amantadine and rimantadine.

To understand how Xofluza works, it is helpful to understand a key step in the influenza virus replication cycle. When the influenza virus infects a cell, it must produce copies of its own genetic material and proteins to create new virus particles. To begin transcribing its RNA genome into messenger RNA (mRNA), the virus uses a strategy called “cap-snatching.” The viral RNA polymerase complex, which includes the cap-dependent endonuclease enzyme, cleaves short capped RNA fragments from the host cell’s own pre-messenger RNA molecules. These stolen cap structures are then used as primers to initiate the transcription of viral mRNA. Without this cap-snatching step, the virus cannot produce the proteins it needs to replicate.

Baloxavir selectively inhibits the CEN enzyme by chelating the two divalent metal ions (manganese or magnesium) in its active site. By blocking cap-snatching, baloxavir prevents the transcription of viral mRNA at a very early stage of the viral lifecycle — before any new viral proteins are made. This is in contrast to neuraminidase inhibitors, which act at the final step of the replication cycle by preventing the release of newly formed virus particles from infected cells. Because baloxavir acts earlier, it produces a more rapid reduction in viral load, which has been consistently demonstrated in clinical studies.

Xofluza is approved for two main indications. First, it is used for the treatment of acute uncomplicated influenza in patients aged 3 weeks and older who have had symptoms for less than 48 hours. The 48-hour window is critical because antiviral therapy is most effective when initiated early in the course of infection, before viral replication has peaked. Second, Xofluza is approved for post-exposure prophylaxis (prevention) of influenza in individuals aged 3 weeks and older who have been in close contact with someone who has confirmed or suspected influenza. For prophylaxis, the single dose should also be taken within 48 hours of exposure.

The clinical efficacy of Xofluza has been established through several pivotal phase III trials. The CAPSTONE-1 trial enrolled 1,436 otherwise healthy patients aged 12–64 years with acute uncomplicated influenza. Patients treated with a single dose of baloxavir experienced a statistically significant reduction in the time to alleviation of flu symptoms (median 53.7 hours) compared with placebo (median 80.2 hours), representing an improvement of approximately 26.5 hours. Importantly, baloxavir also produced a dramatically faster reduction in viral load — the median time to cessation of viral shedding was approximately 24 hours with baloxavir versus 72 hours with placebo.

The CAPSTONE-2 trial focused on patients at high risk for influenza complications, including those aged 65 and older, patients with chronic lung disease (including asthma), patients with metabolic disorders (including diabetes), and immunocompromised individuals. This trial demonstrated that baloxavir reduced the time to improvement of flu symptoms compared with placebo and was non-inferior to oseltamivir. In the high-risk population, baloxavir also reduced the incidence of influenza-related complications, including sinusitis, otitis media, bronchitis, and pneumonia, compared with placebo.

The BLOCKSTONE prophylaxis trial evaluated single-dose baloxavir in household contacts of influenza patients. Among 752 household members randomized to receive either baloxavir or placebo, baloxavir reduced the risk of developing clinical influenza by approximately 86% compared with placebo (1.9% vs 13.6%), confirming its efficacy as a post-exposure prophylactic agent.

Xofluza was first approved in Japan in February 2018 and subsequently by the U.S. FDA in October 2018. The European Medicines Agency (EMA) granted marketing authorization in 2021. It is now available in more than 50 countries worldwide. Xofluza is active against both influenza A (including H1N1 and H3N2 subtypes) and influenza B viruses, and retains activity against strains that are resistant to neuraminidase inhibitors, making it an important addition to the influenza treatment armamentarium.

What Should You Know Before Taking Xofluza?

Contraindications

The primary contraindication to Xofluza is known hypersensitivity (allergy) to baloxavir marboxil or to any of the other excipients in the formulation. If you have previously experienced an allergic reaction to Xofluza or any of its components, you must not take this medication again. Symptoms of hypersensitivity may include skin rash, itching, swelling, or in rare cases, anaphylaxis (a severe, potentially life-threatening allergic reaction).

There are no absolute contraindications based on underlying medical conditions. However, Xofluza has not been studied in certain patient populations, and caution is warranted in specific clinical situations as described below.

Warnings and Precautions

Before taking Xofluza, inform your healthcare provider about all medications, supplements, and over-the-counter products you are currently using. Pay particular attention to the following considerations:

• Timing of treatment: Xofluza must be taken within 48 hours of the onset of influenza symptoms (for treatment) or within 48 hours of close contact with an infected person (for prevention). Starting treatment later than 48 hours significantly reduces the potential benefit.
• Bacterial infections: Xofluza is effective only against influenza viruses. It will not treat bacterial infections, the common cold, or other respiratory infections caused by non-influenza viruses. If your symptoms worsen or do not improve, or if new symptoms develop, consult your doctor as you may have a different or additional infection that requires different treatment.
• Immunocompromised patients: While Xofluza has been studied in immunocompromised patients in the CAPSTONE-2 trial, the data in severely immunocompromised individuals are limited. These patients may experience prolonged viral shedding and are at higher risk for the emergence of drug-resistant variants. Close monitoring is recommended.
• Viral resistance: Emergence of influenza virus variants with reduced susceptibility to baloxavir (primarily through I38T/M substitutions in the PA subunit of the viral polymerase) has been observed in clinical trials, particularly in patients under 12 years of age. The clinical significance of these variants is an area of ongoing research. Resistance rates in clinical trials ranged from approximately 2–10% in adults to up to 23% in children under 12.

Infants and Children

Xofluza is approved for patients aged 3 weeks and older. However, it should not be given to infants under 3 weeks of age, as the effects of baloxavir marboxil have not been studied in this age group. For very young infants, children weighing less than 20 kg should be given the granule formulation for oral suspension rather than the film-coated tablets, as the tablets cannot be appropriately dosed for lower body weights. The tablet formulation is suitable for patients weighing 20 kg or more.

Pregnancy and Breastfeeding

If you are pregnant, think you may be pregnant, or are planning to become pregnant, you should avoid using Xofluza as a precaution. The effects of baloxavir marboxil on human pregnancy have not been adequately studied. Animal reproductive toxicity studies have not shown direct harmful effects on the fetus at clinically relevant doses, but animal studies are not always predictive of human outcomes. For pregnant women with influenza, oseltamivir is generally the preferred antiviral because it has a longer track record and more extensive pregnancy safety data. Consult your doctor to weigh the potential risks and benefits.

It is not known whether baloxavir or its metabolites are excreted in human breast milk. Animal studies have shown that baloxavir acid is excreted in the milk of lactating rats. As a precaution, breastfeeding women should consult their doctor before using Xofluza. The decision to breastfeed during or shortly after treatment should take into account the short duration of drug exposure (single dose) and the potential benefits of treatment for the mother.

Driving and Operating Machinery

Xofluza is unlikely to affect your ability to drive or operate machinery. No specific studies have been performed, but based on its pharmacological profile and known side effects, impairment of driving ability or machine operation is not expected. However, influenza itself can cause fatigue, dizziness, and impaired concentration, so exercise caution if you feel unwell.

Important Information About Ingredients

Xofluza tablets contain lactose monohydrate. If you have been told by your doctor that you have an intolerance to some sugars, contact your doctor before taking this medicine. Xofluza also contains croscarmellose sodium (E468) and sodium stearyl fumarate, but the total sodium content is less than 1 mmol (23 mg) per tablet, meaning it is essentially “sodium-free” and is not a concern for patients on a sodium-restricted diet.

How Does Xofluza Interact with Other Drugs?

Unlike many medications that are metabolized through the cytochrome P450 (CYP) enzyme system, baloxavir acid is primarily metabolized by the UGT1A3 enzyme (uridine diphosphate glucuronosyltransferase). This metabolic pathway means that traditional CYP-mediated drug-drug interactions are not expected with Xofluza. However, there is one critically important interaction that all patients must be aware of — the interaction with polyvalent cation-containing products.

Baloxavir acid functions by chelating (binding to) metal ions in the active site of the CEN enzyme. Unfortunately, this same chelating property means that baloxavir can also bind to polyvalent cations (such as iron, zinc, calcium, magnesium, selenium, and aluminium) present in commonly used medications and supplements. When baloxavir forms chelation complexes with these metal ions in the gastrointestinal tract, it becomes poorly absorbed, leading to significantly reduced blood levels and potentially inadequate antiviral activity.

Pharmacokinetic studies have demonstrated that co-administration of baloxavir with certain polyvalent cation-containing products can reduce the maximum plasma concentration (C_max) and the area under the curve (AUC) of baloxavir acid by approximately 40–50%. This reduction is clinically significant and could result in treatment failure.

It is important to review all your current medications with your doctor or pharmacist before taking Xofluza. Many commonly used over-the-counter products, including antacids for heartburn, mineral supplements, and multivitamins, contain the polyvalent cations that can interfere with Xofluza. If you regularly take any of these products, discuss timing strategies with your healthcare provider. As Xofluza is a single-dose treatment, the most practical approach may be to skip the interfering supplement on the day you take Xofluza.

No clinically significant interactions have been identified between Xofluza and commonly used symptomatic treatments for influenza, including paracetamol (acetaminophen), ibuprofen, and other nonsteroidal anti-inflammatory drugs (NSAIDs). These medications can be safely used alongside Xofluza for fever and pain relief during the flu.

What Is the Correct Dosage of Xofluza?

Always take Xofluza exactly as your doctor or pharmacist has instructed. The dosing of Xofluza is straightforward: it is a single oral dose based on body weight. Unlike most other antiviral medications for influenza, there is no multi-day treatment course — a single dose provides the full therapeutic effect thanks to the long half-life of the active metabolite, baloxavir acid (approximately 79 hours).

Adults and Adolescents (12 years and older)

For treatment of influenza, take the single dose as soon as possible, within 48 hours of the first appearance of influenza symptoms (such as fever, body aches, headache, cough, sore throat, and fatigue). The earlier the dose is taken within this window, the more effective the treatment is likely to be. Xofluza can be taken with or without food — either on an empty stomach or after a meal. Swallow the tablets whole with water.

For prevention of influenza (post-exposure prophylaxis), take the single dose as soon as possible, within 48 hours of close contact with someone who has or is suspected to have influenza. Close contact typically means living in the same household, or having prolonged face-to-face exposure to an infected individual.

Children (3 weeks to under 12 years)

For children weighing 20 kg or more, the same weight-based dosing as for adults applies (40 mg for 20–80 kg). For children weighing less than 20 kg, Xofluza is available as granules for oral suspension, which allows for precise weight-based dosing. The tablet formulation should not be used in patients weighing under 20 kg, as the available tablet strengths do not permit appropriate dose adjustments for lower body weights. Xofluza should not be given to infants under 3 weeks of age.

Elderly Patients

No dose adjustment is required for elderly patients. The same weight-based dosing applies regardless of age. Elderly patients (65 years and older) were included in the CAPSTONE-2 clinical trial and showed comparable efficacy and safety profiles to younger patients. However, elderly patients are often at higher risk for influenza complications and may also be taking multiple medications — it is important to review all current medications, particularly antacids and mineral supplements, to avoid interactions.

Missed Dose

If you have forgotten to take your dose or only taken part of the dose, take it as soon as possible. For treatment purposes, Xofluza should be taken within 48 hours of the onset of influenza symptoms. For prophylaxis, it should be taken within 48 hours of close contact with an infected individual. If you are outside the 48-hour window, consult your doctor about whether treatment is still appropriate for your situation. Since Xofluza is a single-dose medication, there is no ongoing daily dosing schedule to maintain.

Overdose

If you accidentally take more Xofluza than prescribed, contact your doctor or pharmacist for advice. In clinical trials and pharmacokinetic studies, single doses up to 80 mg and supratherapeutic exposures have been evaluated without dose-limiting toxicity. There is no specific antidote for baloxavir marboxil overdose. Treatment should be supportive, based on clinical symptoms and signs, with monitoring as appropriate. Given the long half-life of baloxavir acid (approximately 79 hours), observation may need to continue for an extended period.

What Are the Side Effects of Xofluza?

Like all medicines, Xofluza can cause side effects, although not everyone who takes it will experience them. In the pivotal clinical trials (CAPSTONE-1 and CAPSTONE-2), the overall incidence of adverse events in the baloxavir group was similar to that in the placebo group, indicating that Xofluza is generally well tolerated. The most commonly reported side effects are gastrointestinal in nature.

The safety profile of Xofluza has been evaluated in clinical trials involving more than 2,000 patients of various ages, including otherwise healthy individuals, high-risk patients, and children. Post-marketing pharmacovigilance data from millions of doses administered worldwide (particularly in Japan, where Xofluza was first widely adopted) have further characterized the safety profile and identified rare adverse events not seen during clinical trials.

Adults, Adolescents, and Children (12 years and older)

Children (3 weeks to under 12 years)

The gastrointestinal side effects (diarrhea and vomiting) reported with Xofluza are generally mild and self-limiting, resolving without specific treatment. It is worth noting that influenza itself commonly causes gastrointestinal symptoms, so in some cases it may be difficult to distinguish between drug-related side effects and symptoms of the underlying illness.

Hypersensitivity reactions to Xofluza have been reported in post-marketing experience. Most allergic reactions are mild, presenting as skin rash or urticaria (hives). In very rare cases, severe anaphylactic reactions have occurred, with signs such as facial swelling, difficulty breathing, low blood pressure, and widespread itchy rash. If you experience any of these symptoms after taking Xofluza, seek emergency medical attention immediately.

In clinical trials, the discontinuation rate due to adverse events was very low (comparable to placebo), further confirming the favorable tolerability profile. No signals of hepatotoxicity (liver damage), nephrotoxicity (kidney damage), cardiac adverse events, or neuropsychiatric events have been associated with Xofluza use. The safety profile appears to be consistent across age groups, including elderly patients and those with underlying high-risk conditions.

One area of clinical interest is the emergence of virus variants with reduced susceptibility to baloxavir. While not a “side effect” in the traditional sense, the development of treatment-emergent resistance (primarily I38T substitution in the PA protein of influenza A viruses) has been observed in approximately 2–10% of adult patients and up to 23% of pediatric patients in clinical trials. These variants generally show reduced sensitivity to baloxavir in vitro but remain susceptible to neuraminidase inhibitors. In most cases, the resistant variants were transient and did not appear to significantly impact the clinical outcome. Ongoing surveillance continues to monitor the prevalence and clinical significance of these variants.

How Should You Store Xofluza?

Proper storage of Xofluza helps ensure the medication remains effective and safe to use. Unlike some medications that require refrigeration, Xofluza film-coated tablets can be stored at normal room temperature. Follow these guidelines:

• Temperature: No special temperature storage conditions are required. Store at room temperature, away from excessive heat or cold.
• Moisture protection: Xofluza tablets are moisture-sensitive. Keep them in the original blister packaging until you are ready to take them. Do not transfer tablets to a separate pill container or pill organizer.
• Keep out of reach of children: Store the medication in a secure location where children cannot access it.
• Expiration date: Do not use Xofluza after the expiration date (EXP) printed on the blister pack and outer carton. The expiration date refers to the last day of the stated month.
• Disposal: Do not dispose of medicines in wastewater or household waste. Ask your pharmacist how to dispose of medicines you no longer use. These measures help protect the environment.

Since Xofluza is a single-dose treatment, you will typically only have a small number of tablets. If you are prescribed Xofluza in advance (for example, to have on hand during flu season for post-exposure prophylaxis), ensure it is stored properly in its original packaging until needed, and check the expiration date before taking it.

What Does Xofluza Contain?

Understanding the composition of your medication is important, particularly if you have known allergies or intolerances. Below is a detailed breakdown of the active and inactive ingredients in Xofluza.

Active Ingredient

The active substance is baloxavir marboxil, a small-molecule prodrug. Each 20 mg film-coated tablet contains 20 mg of baloxavir marboxil, and each 40 mg film-coated tablet contains 40 mg of baloxavir marboxil. After oral ingestion, baloxavir marboxil is rapidly hydrolyzed by arylacetamide deacetylase (AADAC) in the gastrointestinal tract and liver to its active form, baloxavir acid, which is the pharmacologically active metabolite responsible for antiviral activity.

Inactive Ingredients (Excipients)

Appearance and Pack Sizes

Xofluza 20 mg tablets are white to pale yellow, oblong film-coated tablets embossed with “▪ 772” on one side and “20” on the other side. Xofluza 40 mg tablets are white to pale yellow, oblong film-coated tablets embossed with “BXM40” on one side. Both strengths are available in blister packs of 2 tablets. The tablets should be kept in the original blister packaging until ready for use to protect from moisture.

Marketing Authorization Holder and Manufacturer

Xofluza is developed by Shionogi & Co., Ltd. (Osaka, Japan) and manufactured by Roche Pharma AG (Grenzach-Wyhlen, Germany). The marketing authorization holder for the European Union is Roche Registration GmbH (Grenzach-Wyhlen, Germany). In the United States, Xofluza is marketed by Genentech, a member of the Roche Group. Xofluza is approved and available in more than 50 countries worldwide, including the EU, US, Japan, and many countries in Asia-Pacific and Latin America.