Pharmacology · Anti-influenza agents
Oseltamivir !
An oral antiviral for influenza A and B. Started within 2 days of symptoms, it shortens the illness; it can also prevent flu after exposure.
Updated 2026-11-01
Also called: oseltamivir phosphate
The figure shows oseltamivir carboxylate, the active form, in the neuraminidase of influenza A (N1).
Mechanism of action
Oseltamivir is an ester prodrug converted in the liver to oseltamivir carboxylate, an analogue of sialic acid. It blocks the active site of the viral neuraminidase, the enzyme that cuts sialic acid from the cell surface. New virions remain attached to the infected cell and clump together, so the virus spreads less.
- Absorbed and converted to oseltamivir carboxylate
- Fits the active site of the viral neuraminidase
- Virions cannot cut sialic acid and stay stuck to the cell
- Spread of the virus to new cells falls
Pharmacokinetics
- Absorption
- Well absorbed by mouth; at least 75% reaches the blood as the carboxylate
- Metabolism
- Ester hydrolysis in the liver to the active carboxylate
- Elimination
- More than 99% of the carboxylate in the urine
- Half-life
- Carboxylate 6 to 10 hours
Indications
- Treatment of acute, uncomplicated influenza A and B in people with symptoms for no more than 48 hours Influenza (flu) →
- Prevention of influenza A and B after exposure or during outbreaks Influenza (flu) →
Adverse effects
Common
- Nausea and vomiting
- Headache
- Pain
Contraindications
- Known serious hypersensitivity to oseltamivir
Key interactions
- Live attenuated (intranasal) influenza vaccine
- The antiviral can stop the vaccine virus from replicating: avoided from 2 weeks before to 48 hours after oseltamivir unless needed; inactivated vaccine can be given at any time
What is monitored
- Kidney function (doses depend on it)
- Behaviour and mental status, especially in children
- Signs of secondary bacterial pneumonia
Administration and safety
- Oral, as capsules or suspension
- Started as soon as possible after symptoms begin
- Not recommended in end-stage kidney disease without dialysis
- It does not replace the annual influenza vaccine
Nursing care
- Note when symptoms started: the benefit is greatest when started early
- Ask families to report unusual behaviour or confusion
- Teach warning signs of complications (shortness of breath, chest pain, persistent fever)
Pregnancy and breastfeeding
Observational studies have not shown more birth defects, although they are small; pregnant women are a high-risk group for severe influenza.
Test yourself
Think of the answer, then tap the question to see it.
What enzyme does oseltamivir carboxylate inhibit?
The influenza virus neuraminidase.
Within what time from symptom onset is oseltamivir indicated for treatment of uncomplicated flu?
Within 48 hours.
Why can oseltamivir interfere with the intranasal flu vaccine?
The intranasal vaccine contains live attenuated virus, whose replication the antiviral can block.
How is oseltamivir mainly eliminated?
As the carboxylate in the urine, so kidney function matters.
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: DailyMed - OSELTAMIVIR capsule (DailyMed (U.S. National Library of Medicine), 2026); Influenza (seasonal) (World Health Organization, 2025); ATCDDD - ATC/DDD Index (WHO Collaborating Centre for Drug Statistics Methodology, 2026); eEML - Electronic Essential Medicines List (World Health Organization, 2025).
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Representative model, for educational purposes only.