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Neuraminidase Inhibitor (Antiviral) Pregnancy: Large post-marketing/observational dataset (over 1000 first-trimester exposures) indicates no malformative or feto/neonatal toxicity; one study's findings on major congenital heart defects were inconclusive. Use may be considered in pregnancy if necessary after weighing safety/benefit and the pathogenicity of the circulating strain.

Oseltamivir

Brand names: Tamiflu

Oseltamivir is an oral antiviral used to treat and prevent influenza, particularly in those at higher risk of complications.

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: 75 mg
Route: Oral
Frequency: Twice daily for 5 days (treatment of influenza)
Infectious diseases context. Adults and adolescents 13 years and over. Start as soon as possible within the first two days of onset of influenza symptoms. Immunocompromised adults/adolescents: 75 mg twice daily for 10 days. Post-exposure prevention after close contact with an infected individual: 75 mg once daily for 10 days, begun within two days of exposure. Prevention during a community influenza epidemic: 75 mg once daily for up to 6 weeks (up to 12 weeks in immunocompromised patients). A 75 mg dose may be given as one 75 mg capsule or one 30 mg plus one 45 mg capsule; oral suspension 6 mg/ml is preferred where capsules cannot be swallowed. Hepatic impairment: no dose adjustment required for treatment or prevention. Renal impairment requires dose adjustment (see renalAdjustment). Children 1-12 years use fixed weight bands: 10-15 kg 30 mg, 15-23 kg 45 mg, 23-40 kg 60 mg, over 40 kg 75 mg - twice daily for treatment, once daily for 10 days for post-exposure prevention.

Paediatric dose

Dose: 3 mg/kg
Route: Oral
Frequency: Twice daily for 5 days (treatment)
SPC per-kg dose applies to infants 0-12 months of age: 3 mg/kg twice daily for treatment (10 days if immunocompromised); post-exposure prevention 3 mg/kg once daily for 10 days. For all patients under 1 year, 3 mg/kg determines the dose regardless of weight. Not intended for premature infants (post-conceptual age under 36 weeks). Children 1-12 years use fixed weight bands rather than a per-kg calculation. Verify against a children's formulary before prescribing.

Dose adjustments

Renal

eMC: dose adjustment is recommended for adults and adolescents (13-17 years) with moderate or severe renal impairment - the creatinine-clearance dosing table in SPC section 4.2 was truncated in the fetched extract, so clinician to confirm against the full SPC. US labelling states: treatment - CrCl >30-60 mL/min 30 mg twice daily for 5 days; CrCl >10-30 mL/min 30 mg once daily for 5 days; ESRD on haemodialysis 30 mg immediately then 30 mg after every dialysis cycle (max 5 days); ESRD on CAPD single 30 mg dose. Prophylaxis - CrCl >30-60 mL/min 30 mg once daily; CrCl >10-30 mL/min 30 mg every other day; ESRD on haemodialysis 30 mg then 30 mg after alternate dialysis cycles; ESRD on CAPD 30 mg then 30 mg once weekly.

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.

Paediatric weight-based calculator

SPC per-kg dose applies to infants 0-12 months of age: 3 mg/kg twice daily for treatment (10 days if immunocompromised); post-exposure prevention 3 mg/kg once daily for 10 days. For all patients under 1 year, 3 mg/kg determines the dose regardless of weight. Not intended for premature infants (post-conceptual age under 36 weeks). Children 1-12 years use fixed weight bands rather than a per-kg calculation. Verify against a children's formulary before prescribing.

Verify in a children's formulary

US labelling (FDA)

Reference — US labelling, may differ from UK

Treatment of influenza Adults and adolescents (13 years and older): 75 mg twice daily for 5 days ( 2.2 ) Pediatric patients 1 to 12 years of age: Based on weight twice daily for 5 days ( 2.2 ) Pediatric patients 2 weeks to less than 1 year of age: 3mg/kg twice daily for 5 days ( 2.2 ) Renally impaired adult patients (creatinine clearance >30 to 60 mL/min): Reduce to 30 mg twice daily for 5 days ( 2.4 ) Renally impaired adult patients (creatinine clearance >10 to 30 mL/min): Reduce to 30 mg once daily for 5 days ( 2.4 ) ESRD patients on hemodialysis: Reduce to 30 mg immediately and then 30 mg after every hemodialysis cycle. Treatment duration not to exceed 5 days ( 2.4 ) ESRD patients on …

Source: US FDA prescribing information (openFDA / DailyMed), label dated 2024-05-15. Accessed 2026-06-12. US dosing and indications can differ from UK practice — use UK sources for prescribing decisions.

Contraindications

  • Hypersensitivity to the active substance or to any of the excipients

Side effects

  • Nausea and vomiting (most common in adults/adolescents on treatment; usually on day 1-2 and resolving spontaneously within 1-2 days)
  • Nausea (most common in prevention studies)
  • Vomiting (most common reaction in children)
  • Bronchitis, nasopharyngitis, upper respiratory tract infection, sinusitis, herpes simplex reported in trials; thrombocytopenia
  • Rare post-marketing: anaphylactic/anaphylactoid reactions, hepatic disorders (fulminant hepatitis, jaundice), angioneurotic oedema, Stevens-Johnson syndrome, toxic epidermal necrolysis, gastrointestinal bleeding, neuropsychiatric disorders

Interactions

  • Live attenuated influenza vaccine (LAIV, intranasal): avoid administration within 2 weeks before or 48 hours after oseltamivir unless medically indicated (US labelling; eMC section 4.5 not captured in fetched extract)
  • Inactivated influenza vaccine may be given at any time relative to oseltamivir (US labelling)
  • No dose adjustment needed with amoxicillin, paracetamol, aspirin, cimetidine, antacids, rimantadine, amantadine or warfarin (US labelling)

Clinical monograph

How it works

It inhibits the viral neuraminidase enzyme, preventing release of new virus particles from infected cells.

Prescribing in practice

  • It is most effective when started early — ideally within 48 hours of symptom onset (or of exposure for prophylaxis).
  • Nausea and vomiting are the commonest effects (taking it with food helps).
  • Reduce the dose in renal impairment.

Monitoring

No routine monitoring; review the clinical response.

Counselling the patient

  • Start it as soon as possible after symptoms begin.
  • Take it with food to reduce nausea.
  • It shortens illness but is not a substitute for vaccination.

Evidence & guidelines

Recommended for treatment and post-exposure prophylaxis of influenza in at-risk groups during circulating influenza (NICE TA158; UKHSA guidance).

Reference: NICE NG172; PHE Influenza Seasonal Report; MHRA Drug Safety; 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).

Related

Curated clinical cross-links plus same-class fallbacks.

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