Neuraminidase inhibitor 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. Breast-feeding: levels in milk are low and would give a subtherapeutic infant dose; administration may be considered where benefits to the mother are clear.

Oseltamivir

Brand names: Tamiflu

An oral neuraminidase inhibitor antiviral used for the treatment and post-exposure prophylaxis of influenza A and B.

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)

Adults and adolescents 13 years and over. Treatment should be initiated 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: 75 mg once daily for 10 days, begun as soon as possible within two days of exposure to an infected individual. 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 or lower doses are needed. Hepatic impairment: no dose adjustment required for treatment or prevention. Children 1-12 years are dosed by weight band, not per kg: 10-15 kg 30 mg, 15-23 kg 45 mg, 23-40 kg 60 mg, over 40 kg 75 mg - twice daily for treatment (5 days; 10 days if immunocompromised) and once daily for 10 days for post-exposure prevention. Community-epidemic prevention has not been studied in children below 12 years.

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), and 3 mg/kg once daily for 10 days for post-exposure prevention. For all patients under 1 year, 3 mg/kg is used to determine the dose regardless of weight. Not intended for premature infants (post-conceptual age under 36 weeks) - insufficient data. Children 1-12 years use fixed weight bands (see adult dose notes), not 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 reduce to 30 mg twice daily for 5 days; CrCl >10-30 mL/min reduce to 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

Patient weight (kg)

Verify in a children's formulary

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)
Headache and other reactions listed in SPC Table 1; bronchitis, nasopharyngitis, upper respiratory tract infection, sinusitis, herpes simplex reported in trials
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

Its active metabolite inhibits the influenza neuraminidase enzyme, preventing release of newly formed virions from infected cells and limiting spread of the virus.

Prescribing in practice

It is most effective when started as early as possible and within a short window of symptom onset, so prompt initiation in at-risk patients is key.
The dose requires reduction in renal impairment as it is renally cleared — check renal function and consult the SPC.
Neuropsychiatric events, including abnormal behaviour, have been reported particularly in children and adolescents, so advise carers to monitor for unusual behaviour.

Monitoring

No routine laboratory monitoring is needed beyond assessing renal function to guide dosing and watching for neuropsychiatric symptoms.

Counselling the patient

Start the medicine as soon as possible after symptoms begin and complete the full course.
It does not replace annual influenza vaccination.
Watch for any confusion, agitation or unusual behaviour, especially in children, and seek advice if this occurs.

Evidence & guidelines

Oseltamivir is recommended by NICE for the treatment and prophylaxis of influenza in at-risk groups during periods of circulating virus.

Reference: NICE TA168/TA158; UKHSA; 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.

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