What is Oseltamivir (Tamiflu)?
Oseltamivir is a neuraminidase inhibitor antiviral medication used to treat and prevent influenza A and B virus infections. 1, 2
Primary Indications
Treatment of Acute Influenza
- Treatment of acute, uncomplicated influenza A and B in patients ≥2 weeks of age who have been symptomatic for no more than 48 hours. 2
- Oseltamivir reduces illness duration by approximately 1–1.5 days (24–36 hours) when started within 48 hours of symptom onset. 1, 3
- The medication reduces the severity of illness by up to 38% and decreases the risk of complications including secondary bacterial infections requiring antibiotics by 34–44%. 1, 3
Prophylaxis (Prevention)
- Prevention of influenza A and B in patients ≥1 year of age following exposure to an infected individual. 2
- When used for seasonal prophylaxis, oseltamivir prevents 74–87% of laboratory-confirmed influenza infections compared to placebo. 3, 4
- Post-exposure prophylaxis in household contacts reduces infection risk by approximately 6.6% in absolute terms when started within 48 hours of the index case's symptom onset. 5
Standard Dosing Regimens
Adults & Adolescents (≥13 years)
- Treatment: 75 mg orally twice daily for 5 days. 1, 2
- Prophylaxis: 75 mg once daily for 10 days (post-exposure) or up to 6 weeks (community outbreak). 1, 2
Pediatric Patients (1–12 years) – Weight-Based
| Body Weight | Treatment Dose (BID × 5 days) | Prophylaxis Dose (QD × 10 days) |
|---|---|---|
| ≤15 kg | 30 mg | 30 mg |
| >15–23 kg | 45 mg | 45 mg |
| >23–40 kg | 60 mg | 60 mg |
| >40 kg | 75 mg | 75 mg |
Infants (2 weeks to <1 year)
- Term infants 0–8 months: 3 mg/kg per dose twice daily for 5 days. 6, 2
- Infants 9–11 months: 3.5 mg/kg per dose twice daily for 5 days. 6, 2
- Preterm infants: Dosing based on postmenstrual age (PMA = gestational age + chronologic age):
- <38 weeks PMA: 1.0 mg/kg twice daily
- 38–40 weeks PMA: 1.5 mg/kg twice daily
40 weeks PMA: 3.0 mg/kg twice daily 6
Renal Dose Adjustments
Creatinine Clearance 10–30 mL/min
- Treatment: 75 mg once daily (instead of twice daily) for 5 days, OR 30 mg once daily for 5 days. 1, 6, 2
- Prophylaxis: 30 mg once daily for 10 days, OR 75 mg every other day for 10 days (5 total doses). 1, 6, 2
End-Stage Renal Disease (ESRD)
- On hemodialysis: 30 mg immediately, then 30 mg after every hemodialysis cycle (treatment) or after alternate cycles (prophylaxis). 2
- On CAPD: Single 30 mg dose immediately (treatment) or 30 mg once weekly (prophylaxis). 2
- Not on dialysis: Oseltamivir is not recommended for ESRD patients not undergoing dialysis. 2
Formulations & Administration
- Capsules: 30 mg, 45 mg, and 75 mg strengths. 6, 2
- Oral suspension: 6 mg/mL concentration after reconstitution (360 mg powder in 60 mL bottle). 6, 2
- Administration with food significantly reduces nausea and vomiting (the most common adverse effects, occurring in 10–15% of patients). 6, 3, 4
- If commercial suspension is unavailable, capsules may be opened and mixed with sweetened liquid to achieve 6 mg/mL concentration. 6
Timing & Clinical Effectiveness
- Optimal window: Initiate treatment within 48 hours of symptom onset for maximal benefit; starting within 12–24 hours provides the greatest reduction in illness duration. 1, 6, 3
- Late initiation: Treatment begun after 48 hours may still benefit high-risk patients (children <2 years, elderly ≥65 years, pregnant women, immunocompromised, chronic disease). 6, 7
- Complete the full 5-day course even if symptoms resolve earlier to prevent viral resistance and ensure adequate viral suppression. 6
Activity Against Influenza B
- Oseltamivir has activity against both influenza A and B viruses, though clinical effectiveness may be somewhat lower against influenza B. 1, 8, 9
- In pediatric studies, oseltamivir reduced illness duration in influenza B from 173.9 to 110.0 hours (p=0.03), though the effect was less pronounced than with influenza A. 10
- The 50% inhibitory concentration against influenza B virus (75.4 nmol/L) is substantially higher than for influenza A (H3N2) virus (0.3 nmol/L), explaining the reduced clinical effectiveness. 10
- Despite lower in vitro potency, oseltamivir remains a primary recommended antiviral for influenza B treatment and prophylaxis, with rare resistance (<1.6%). 8, 10
Contraindications
- Known serious hypersensitivity to oseltamivir or any component of the formulation. 2
- Serious skin/hypersensitivity reactions (Stevens-Johnson syndrome, toxic epidermal necrolysis, erythema multiforme) require immediate discontinuation. 2
Important Safety Information
Common Adverse Effects
- Nausea and vomiting (10–15% of patients) are mild, transient, and significantly reduced when taken with food. 6, 3, 4
- Only approximately 1% of patients discontinue oseltamivir due to gastrointestinal side effects. 6
Neuropsychiatric Events
- Patients with influenza, particularly pediatric patients, may be at increased risk of confusion or abnormal behavior early in illness. 2
- Controlled clinical trials and ongoing surveillance have failed to establish a causal link between oseltamivir and neurologic or psychiatric events. 7
Drug Interactions
- Live attenuated influenza vaccine (LAIV): Avoid LAIV administration within 2 weeks before or 48 hours after oseltamivir use, unless medically indicated. 2
- Oseltamivir does not interfere with the antibody response to inactivated influenza vaccine (IIV). 1
Critical Clinical Pitfalls
- Do not delay treatment while awaiting laboratory confirmation in high-risk patients or during influenza season. 6, 7
- Do not use prophylactic once-daily dosing to treat symptomatic patients; switch immediately to twice-daily treatment dosing. 6
- Do not apply term-infant dosing to preterm infants; use postmenstrual age-based dosing to avoid toxic accumulation. 6
- Do not withhold treatment in high-risk patients presenting after 48 hours; they still benefit from antiviral therapy. 6, 7
- Do not use household spoons for liquid formulation; always use a calibrated oral syringe (3 mL or 5 mL). 6
Limitations of Use
- Oseltamivir is not a substitute for annual influenza vaccination, which remains the primary preventive measure. 1, 2
- Consider available information on influenza drug susceptibility patterns and treatment effects when deciding whether to use. 2
- Oseltamivir has not been demonstrated to prevent serious influenza-related complications such as bacterial pneumonia or exacerbation of chronic diseases in controlled trials. 1