ClinCalc Pro
Menu
Calculators Drugs Pathways AI Dx Donate Get the App
Influenza Antiviral — Cap-Dependent Endonuclease Inhibitor Pregnancy: Insufficient data — animal studies show no reproductive toxicity at clinical doses. Use only if benefit clearly outweighs risk; oseltamivir has more pregnancy safety data and is preferred in pregnancy.

Baloxavir Marboxil

Brand names: Xofluza

Adult dose

Dose: <80 kg: 40 mg as single dose. ≥80 kg: 80 mg as single dose. High-risk adults: 40 mg or 80 mg on day 1, then same dose on day 5 (2-dose regimen)
Route: Oral
Frequency: Single dose (or two doses 5 days apart in high-risk patients)
Max: 80 mg per dose
Influenza A and B in adults and children ≥1 year. Start within 48 hours of symptom onset (ideally within 24 hours). Single dose is a major adherence advantage vs oseltamivir (5-day course). High-risk groups (elderly, immunocompromised): use two-dose regimen. Available as 20 mg and 40 mg tablets. Source: BNF 90; MHRA SPC Xofluza.

Paediatric dose

Dose: Age ≥1 year: <20 kg: 10 mg; 20–<40 kg: 20 mg; 40–<80 kg: 40 mg; ≥80 kg: 80 mg (all as single dose) mg (weight-banded)/kg
Route: Oral
Frequency: Single dose
Max: 80 mg
Licensed from 1 year. Weight-banded single dosing. Paediatric tablet or oral suspension available. Source: BNF for Children 2024.

Dose adjustments

Renal

No dose adjustment required for renal impairment. Baloxavir is metabolised hepatically and renally excreted minimally.

Hepatic

Severe hepatic impairment: not recommended — limited data. Mild-moderate: no adjustment.

Paediatric weight-based calculator

Licensed from 1 year. Weight-banded single dosing. Paediatric tablet or oral suspension available. Source: BNF for Children 2024.

Clinical pearls

  • Completely different mechanism from oseltamivir: oseltamivir inhibits neuraminidase (blocks viral release from infected cells). Baloxavir inhibits cap-dependent endonuclease (PA subunit of RNA polymerase) — blocks viral mRNA transcription completely. No cross-resistance. Active against oseltamivir-resistant influenza strains (H275Y mutation).
  • CAPSTONE-1 trial (NEJM 2018): baloxavir single dose was non-inferior to oseltamivir 5-day course for time to alleviation of symptoms in uncomplicated influenza (median 53.7h vs 53.8h). Significantly greater reduction in viral load at 24h post-dose — baloxavir more rapidly reduces viral replication (mechanism: stops transcription vs oseltamivir: stops release).
  • Resistance concern: PA-I38X mutations confer baloxavir resistance and emerge during treatment (detected in 2–10% of treated patients — higher in children). Clinical significance uncertain — may reduce efficacy in severe disease. Surveillance ongoing. Combination with oseltamivir is being studied for high-risk patients.
  • Single-dose convenience advantage: complete influenza treatment in one tablet vs 5-day oseltamivir course. Particularly valuable in patients with adherence difficulties, elderly patients, or as post-exposure prophylaxis (single dose) in household contacts.
  • Cation chelation — the most important interaction: milk, antacids, iron supplements, zinc lozenges, and magnesium-containing preparations chelate baloxavir and reduce absorption by up to 75%. Must be taken with water, NOT dairy, and separated from any cation-containing product by at least 2 hours (6 hours preferred). Source: BNF 90; Hayden et al. NEJM 2018 (CAPSTONE-1); MHRA SPC Xofluza.

Contraindications

  • Polyvalent cation-containing products at time of baloxavir dose (calcium, magnesium, iron, zinc — chelation reduces absorption by 75%)
  • Hypersensitivity to baloxavir

Side effects

  • Diarrhoea, nasopharyngitis, headache, nausea (most common — generally mild)
  • Bronchitis (similar frequency to placebo — influenza sequelae)
  • Elevated liver transaminases (monitor if prolonged use or repeated courses)

Interactions

  • Polyvalent cations (antacids containing Ca/Mg/Al, iron supplements, zinc, calcium supplements, laxatives): chelate baloxavir and reduce absorption by 75% — take baloxavir at least 2 hours before or 6 hours after any polyvalent cation-containing product
  • Influenza vaccines: live attenuated influenza vaccine (LAIV) should be avoided for 14 days after baloxavir (antiviral suppresses vaccine viral replication). Inactivated vaccine — no interaction

Monitoring

  • Symptom diary (influenza course — time to alleviation)
  • Fever resolution (expected within 24–48h)
  • Adverse events (diarrhoea, headache)
  • In high-risk patients: repeat dose adherence at day 5

Reference: BNFc; BNF 90; BNF for Children 2024; Hayden et al. NEJM 2018 (CAPSTONE-1); Ison et al. NEJM 2020 (CAPSTONE-2); MHRA SPC Xofluza. Verify against your local formulary and the latest BNF before prescribing.

Related

Curated clinical cross-links plus same-class fallbacks.

Calculators
Pathways