Pharmacology · Anti-influenza agents
Baloxavir marboxil !
A single-dose oral flu antiviral that blocks the virus's «cap-snatching» enzyme. Milk and mineral supplements reduce its absorption.
Updated 2026-11-01
The figure shows baloxavir acid, the active form, in the endonuclease domain of the PA protein of influenza A (H1N1, 2009 pandemic strain).
Mechanism of action
Baloxavir marboxil is a prodrug converted to baloxavir acid. Influenza starts making its mRNA by cutting the cap off host mRNAs («cap-snatching») with the endonuclease in its PA subunit. Baloxavir acid binds this endonuclease and the metal ions in its active site, so viral mRNA synthesis cannot begin. Substitutions at position I38 of PA reduce susceptibility.
- Converted to baloxavir acid after absorption
- Binds the PA endonuclease and its metal ions
- The virus cannot steal caps from host mRNA
- Viral mRNA and protein synthesis cannot start
Pharmacokinetics
- Absorption
- Oral; polyvalent cations (calcium, magnesium, aluminium, iron, zinc, selenium) chelate it and reduce absorption
- Metabolism
- Hydrolysed to the active baloxavir acid
Indications
- Treatment of acute uncomplicated influenza early in the illness Influenza (flu) →
- Prevention of influenza after exposure Influenza (flu) →
Adverse effects
Common
- Diarrhoea
- Nausea
- Vomiting
Contraindications
- Hypersensitivity to baloxavir marboxil
Key interactions
- Dairy products, calcium-fortified drinks
- Reduce absorption; avoided around the dose
- Antacids, laxatives and supplements with calcium, magnesium, aluminium, iron, zinc or selenium
- Bind the drug and reduce its levels
What is monitored
- Clinical response and signs of complications
- Allergic reactions after the dose
Administration and safety
- Single oral dose, with or without food, but not with dairy or calcium-fortified drinks
- Ask about mineral supplements and antacids before giving it
Nursing care
- Review supplements, antacids and laxatives before the dose
- Teach to avoid milk and yogurt around the dose
- Teach warning signs of complications of influenza
Pregnancy and breastfeeding
Human data are limited; the benefit and risk are weighed.
Test yourself
Think of the answer, then tap the question to see it.
What does baloxavir inhibit?
The cap-dependent endonuclease of the PA subunit of the influenza polymerase («cap-snatching»).
Why is baloxavir not given with milk or antacids?
Calcium, magnesium and other polyvalent cations bind it and reduce absorption.
Which PA substitution reduces susceptibility to baloxavir?
Changes at isoleucine 38 (I38T and others).
Related drugs
Related conditions
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Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.
Sources: Baloxavir Marboxil: MedlinePlus Drug Information (MedlinePlus (U.S. National Library of Medicine), 2026); ATCDDD - ATC/DDD Index (WHO Collaborating Centre for Drug Statistics Methodology, 2026); Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th ed., ch. 62 (McGraw Hill, 2023).
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Representative model, for educational purposes only.