Oseltamivir (Tamiflu): Comprehensive Clinical Guide
Treatment Dosing
Adults and Adolescents ≥13 Years
For treatment of influenza, administer oseltamivir 75 mg orally twice daily for 5 days. 1
Pediatric Dosing (≥12 months)
Weight-based dosing for treatment (twice daily for 5 days): 1, 2
- ≤15 kg: 30 mg twice daily
- >15-23 kg: 45 mg twice daily
- >23-40 kg: 60 mg twice daily
- >40 kg: 75 mg twice daily
Infants <12 Months
- 9-11 months: 3.5 mg/kg/dose twice daily 1, 2
- 0-8 months (term infants): 3 mg/kg/dose twice daily 1
- Note: Oseltamivir is not FDA-approved for infants <2 weeks, but may be used in critical situations 1
Prophylaxis Dosing
Post-Exposure Prophylaxis (10 days)
Adults: 75 mg once daily 1
Pediatric (weight-based, once daily): 1, 2
- ≤15 kg: 30 mg once daily
15-23 kg: 45 mg once daily
23-40 kg: 60 mg once daily
40 kg: 75 mg once daily
Extended Prophylaxis
- Institutional outbreaks: Continue for minimum 2 weeks or until 7-10 days after last case 1
- Severely immunocompromised patients: May extend up to 12 weeks during community outbreaks 3
Renal Dose Adjustments
Creatinine Clearance 10-30 mL/min
Creatinine Clearance >30-60 mL/min
- Prophylaxis only: 30 mg once daily 3
End-Stage Renal Disease
Critical Timing Considerations
Optimal Treatment Window
Initiate oseltamivir immediately within 48 hours of symptom onset for maximum benefit—this reduces illness duration by 1-1.5 days in healthy adults and 17.6-29.9 hours in children. 1, 3, 4, 5
- Within 12 hours: Reduces illness duration by additional 74.6 hours compared to 48-hour initiation 4
- Within 24 hours: Reduces illness duration by additional 53.9 hours compared to 48-hour initiation 4
Treatment Beyond 48 Hours—High-Risk Populations
Do not withhold oseltamivir in high-risk or severely ill patients presenting after 48 hours, as substantial mortality benefit persists when initiated up to 96 hours after symptom onset (OR for death = 0.21). 3, 6
High-risk populations who benefit from late treatment include: 3, 6
- Children <2 years (especially <6 months)
- Adults ≥65 years
- Pregnant or postpartum women
- Immunocompromised patients (including those on long-term corticosteroids)
- Patients with chronic cardiac, pulmonary, renal, hepatic, or neurological disease
- Patients with diabetes requiring medication
- All hospitalized patients with suspected influenza
Healthy Outpatients Beyond 48 Hours
In previously healthy outpatients without high-risk features who are not deteriorating, do not initiate oseltamivir after 48 hours—supportive care alone is appropriate. 3
Clinical Benefits
Symptom Reduction
- Illness duration: Shortened by 1-1.5 days when started within 48 hours 3, 6, 5
- Symptom severity: Reduced by 30-38% 3
Complication Prevention
- Pneumonia risk: 50% reduction 3, 6
- Otitis media in children: 34% reduction 3, 6
- Secondary bacterial infections requiring antibiotics: 35% reduction 3
Mortality Benefit
- Hospitalized patients: Significantly decreased risk of death within 15 days (OR = 0.21) 3, 6
- Benefit persists even when treatment started >48 hours after onset 3, 6
Viral Shedding
Contraindications and Precautions
Absolute Contraindications
- Known hypersensitivity to oseltamivir or any component 1
Special Populations
Pregnancy: Benefits outweigh risks—treat pregnant women with suspected influenza 3
Elderly (≥65 years): No dose reduction needed based on age alone; exposure to active metabolite is ~25% higher but not clinically significant 1, 8
Hereditary fructose intolerance: Oseltamivir suspension contains sorbitol, which may cause dyspepsia and diarrhea 3
Adverse Effects
Common (Gastrointestinal)
- Nausea: 3.66% increased risk (NNTH = 28) 3
- Vomiting: 4.56-5.34% increased risk (NNTH = 19-22); occurs in ~15% of children vs 9% on placebo 1, 3, 4
- Diarrhea: May occur, especially in infants <1 year 3
Mitigation strategy: Taking oseltamivir with food significantly reduces nausea and vomiting 1, 3, 4, 5
Neuropsychiatric Events
No established causal link between oseltamivir and neuropsychiatric events has been confirmed, though monitoring is recommended. 3
Administration and Formulations
Available Forms
Suspension Dosing Volumes (6 mg/mL)
Compounding
If commercial suspension unavailable, retail pharmacies can compound oseltamivir to 6 mg/mL concentration using capsule contents mixed with simple syrup or Ora-Sweet SF. 1, 2
Alternative Antiviral: Zanamivir
Indications
- Treatment: Approved for children ≥7 years; 10 mg (two 5-mg inhalations) twice daily 1
- Prophylaxis: Approved for children ≥5 years; 10 mg once daily 1
Contraindications
Zanamivir is not recommended for patients with underlying airways disease (asthma, COPD) due to risk of bronchospasm. 1
Renal Impairment
No dose adjustment recommended for zanamivir, even with severe renal impairment 1
Resistance Considerations
Use zanamivir if oseltamivir resistance is suspected or confirmed (though resistance remains <5% in the United States). 3, 6
Critical Clinical Pitfalls to Avoid
Do Not Wait for Laboratory Confirmation
Initiate treatment empirically in high-risk patients based on clinical suspicion during influenza season—rapid antigen tests have poor sensitivity (~50-70%), and negative results should not exclude treatment. 3, 6
Do Not Withhold Based on Time Alone in High-Risk Patients
The 48-hour guideline represents optimal benefit, not an absolute contraindication to later treatment in children <2 years, hospitalized patients, or those with severe/progressive illness. 3, 6
Do Not Round Up Weight Categories
A child weighing 15.2 kg receives 30 mg (not 45 mg); a child weighing 19.5 kg receives 45 mg (not 60 mg). 2
Do Not Use as Substitute for Vaccination
Annual influenza vaccination remains the primary prevention strategy; oseltamivir is adjunctive, not a replacement. 3, 5
Do Not Reflexively Add Antibiotics
Influenza is viral—only add antibiotics if new consolidation on imaging, purulent sputum, clinical deterioration despite oseltamivir, or elevated inflammatory markers suggest bacterial superinfection (most commonly S. pneumoniae, S. aureus, H. influenzae). 3, 6
Institutional Outbreak Management
Nursing Homes and Long-Term Care Facilities
Administer chemoprophylaxis to all eligible residents regardless of vaccination status; continue for minimum 2 weeks or until 7-10 days after last case. 1
Offer prophylaxis to unvaccinated staff caring for high-risk patients. 1
Consider prophylaxis for all employees (regardless of vaccination) if outbreak caused by vaccine-mismatched strain. 1
Infection Control Measures
Reduce contact between persons taking oseltamivir for treatment and those taking prophylaxis to limit transmission of drug-resistant virus. 1
Pharmacokinetics and Drug Interactions
Absorption and Metabolism
- Bioavailability: 80% of active metabolite after oral administration 8
- Time to peak: 3-4 hours 8
- Half-life: 6-10 hours 8
- Elimination: Primarily renal (>90% as active metabolite) 8
Drug Interactions
No clinically significant interactions with acetaminophen, cimetidine, or most medications—oseltamivir does not interact with cytochrome P450 enzymes or glucuronosyltransferases. 8
Probenecid increases oseltamivir exposure 2.5-fold via competition for renal tubular secretion, but this is unlikely to be clinically relevant. 8
Resistance Considerations
Prevalence
Oseltamivir resistance remains low (<5% in the United States) and is rarely of clinical significance due to reduced transmissibility and pathogenicity of resistant mutants. 3, 4, 9
Resistance may be more common in children (up to 18% in one study) than adults. 3
Influenza Type Differences
Oseltamivir demonstrates greater efficacy against influenza A (34% reduction in time to resolution) compared to influenza B (8.5% reduction). 3