Cap-dependent endonuclease inhibitor (antiviral) Pregnancy: eMC §4.6: there are no or limited data from the use of baloxavir marboxil in pregnant women; animal studies do not indicate direct or indirect harmful effects with respect to reproductive toxicity. As a precautionary measure, it is preferable to avoid the use of Xofluza during pregnancy. Breast-feeding: it is unknown whether baloxavir marboxil or baloxavir are excreted in human milk, but baloxavir marboxil and its metabolites are secreted in the milk of lactating rats and a risk to newborns/infants cannot be excluded — a decision must be made whether to discontinue breast-feeding or to abstain from therapy.

Baloxavir marboxil

Brand names: Xofluza

Baloxavir marboxil is an orally active prodrug antiviral used as a single-dose treatment of uncomplicated influenza A and B, and for post-exposure prophylaxis in eligible contacts.

Auto-extracted from the source labelling — not yet independently clinician-verified. These values were distilled from the UK SPC (or the US label where noted) but have not had a clinician sign-off. Confirm against the current SmPC before prescribing.

Adult dose

Dose: A single oral dose determined by body weight: 20 kg or more to less than 80 kg — 40 mg (1 × 40 mg tablet); 80 kg or more — 80 mg (1 × 80 mg tablet or 2 × 40 mg tablets)

Route: Oral — tablets should be taken with water, with or without food. Should not be taken with products that contain polyvalent cations such as laxatives, antacids, or oral supplements containing iron, zinc, selenium, calcium or magnesium.

Frequency: Single dose. For treatment of influenza, take as soon as possible within 48 hours of symptom onset. For post-exposure prophylaxis of influenza, take as soon as possible within 48 hours following close contact with an individual known or suspected to have influenza.

eMC §4.2 (Xofluza 40 mg film-coated tablets). The same single weight-based dose applies to adults, adolescents, children and infants aged 3 weeks and older. REPEAT DOSING: there are no clinical data on the use of a repeat dose of baloxavir marboxil for the treatment of uncomplicated influenza or for post-exposure prophylaxis in any one influenza season. PATIENTS UNDER 20 kg: refer to the Xofluza granules for oral suspension prescribing information — the tablet SPC gives no dose for this weight band. Adults, adolescents and children who are unable to, or experience difficulty, swallowing tablets, or those who require enteral administration, may instead receive Xofluza granules for oral suspension. ELDERLY: no dose adjustment required. HEPATIC IMPAIRMENT: no dose adjustment required in mild or moderate impairment (Child-Pugh class A or B); safety and efficacy have not been established in severe hepatic impairment (Child-Pugh class C). PAEDIATRIC: safety and efficacy in preterm neonates and children aged under 3 weeks have not been established and no data are available; verify all under-18 dosing against a children's formulary. US LABELLING DIFFERENCE (openFDA, Xofluza — for awareness only, verify against the UK SPC before acting on it): the US label restricts Xofluza to patients 5 years of age and older, stating that it 'is not indicated in patients less than 5 years of age due to increased incidence of treatment-emergent resistance in this age group', and gives an oral suspension (bottles, 2 mg/mL) dose of 2 mg/kg for patients weighing less than 20 kg, a 30 mg packet for 15 kg to less than 20 kg, 40 mg (20 mL) for 20 kg to less than 80 kg and 80 mg (40 mL) for 80 kg or more. The UK SPC fetched here does not contain a per-kg dose and defers weights under 20 kg to the separate granules prescribing information.

Dose adjustments

Renal

eMC §4.2: no dose adjustment is required in patients with renal impairment.

Dose auto-extracted from UK Summary of Product Characteristics (SPC) via the eMC; US FDA prescribing information (openFDA / DailyMed) — cross-check; US labelling may differ from UK — not yet clinician-verified. Always confirm against the product SmPC and your local formulary before prescribing.

Contraindications

Hypersensitivity to the active substance or to any of the excipients (eMC §4.3)

Side effects

Diarrhoea (common)
Vomiting (common)
Urticaria (uncommon)
Anaphylaxis, anaphylactic reactions, hypersensitivity and angioedema (frequency not known — identified from post-marketing experience, not reported in clinical studies)
Paediatric patients (3 weeks to under 12 years) in clinical trials: diarrhoea, vomiting and rash (all common)

Interactions

Products that contain polyvalent cations may decrease plasma concentrations of baloxavir — do not take Xofluza with laxatives, antacids or oral supplements containing iron, zinc, selenium, calcium or magnesium (eMC §4.5)
Influenza vaccines — interaction studies with influenza vaccines and baloxavir marboxil have not been conducted; in studies of naturally acquired influenza, treatment with Xofluza did not impair the humoral antibody response to influenza infection
eMC §4.5 notes that interaction studies have only been performed in adults

Clinical monograph

How it works

Its active metabolite baloxavir inhibits the cap-dependent endonuclease activity of the influenza polymerase acidic protein, blocking viral mRNA synthesis and so halting viral replication.

Prescribing in practice

Greatest benefit occurs when started within the first day or two of symptom onset, so confirm timing of illness before prescribing.
Avoid co-administration with polyvalent cation-containing products such as antacids, oral magnesium, aluminium, calcium or iron preparations and dairy, as chelation markedly reduces absorption.
Treatment-emergent reduced-susceptibility variants can arise, which is relevant in immunocompromised or prolonged illness.

Monitoring

Routine laboratory monitoring is not required; monitor for symptom resolution and watch for rare hypersensitivity reactions.

Counselling the patient

This is usually a single oral dose taken as soon as possible after flu symptoms begin.
Do not take it at the same time as antacids, dairy, or iron or calcium supplements.
Antiviral treatment does not replace annual influenza vaccination.

Evidence & guidelines

The CAPSTONE-1 and CAPSTONE-2 trials demonstrated reduced time to symptom resolution in uncomplicated influenza.

Reference: NICE NG34; UKHSA influenza; SmPC; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Curated clinical cross-links plus same-class fallbacks.

Calculators

Pathways