Oseltamivir Mechanism of Action and Dosing
Oseltamivir is an oral neuraminidase inhibitor that blocks viral replication by preventing influenza virions from being released from infected host cells, and it must be initiated within 48 hours of symptom onset to achieve clinical benefit. 1, 2
Mechanism of Action
Oseltamivir functions as a prodrug that is rapidly converted by hepatic esterases into its active metabolite, oseltamivir carboxylate, which selectively inhibits influenza A and B neuraminidase enzymes essential for viral replication 3, 4. The drug achieves high bioavailability and penetrates infection sites at concentrations sufficient to inhibit viral replication 4. By blocking neuraminidase, oseltamivir prevents the liberation of new viral particles from infected cells, making it effective only during the active replication phase 2.
Treatment Dosing
Adults and Adolescents (≥13 years)
Pediatric Patients (≥12 months) - Weight-Based Dosing
- ≤15 kg (≤33 lb): 30 mg twice daily for 5 days 5, 6
- >15-23 kg (33-51 lb): 45 mg twice daily for 5 days 5, 6
- >23-40 kg (51-88 lb): 60 mg twice daily for 5 days 5, 6
- >40 kg (>88 lb): 75 mg twice daily for 5 days 5, 6
Infants (2 weeks to <12 months)
- 9-11 months: 3.5 mg/kg per dose twice daily for 5 days 5, 6
- Term infants 0-8 months: 3 mg/kg per dose twice daily for 5 days 5, 6
Preterm Infants (Based on Postmenstrual Age)
- <38 weeks postmenstrual age: 1.0 mg/kg twice daily 6, 7
- 38-40 weeks postmenstrual age: 1.5 mg/kg twice daily 6, 7
- >40 weeks postmenstrual age: 3.0 mg/kg twice daily 6, 7
Prophylaxis Dosing
Adults and Adolescents (≥13 years)
- 75 mg orally once daily for at least 10 days following close contact with an infected individual 5, 1
- Up to 6 weeks during community outbreaks 5, 1
- Up to 12 weeks in immunocompromised patients 1
Pediatric Patients (≥1 year) - Weight-Based Dosing
- ≤15 kg: 30 mg once daily for 10 days 5, 6
- >15-23 kg: 45 mg once daily for 10 days 5, 6
- >23-40 kg: 60 mg once daily for 10 days 5, 6
- >40 kg: 75 mg once daily for 10 days 5, 6
Renal Impairment Adjustments
For patients with creatinine clearance 10-30 mL/min:
- Treatment: 75 mg once daily for 5 days 5, 6, 7
- Prophylaxis: 30 mg once daily for 10 days OR 75 mg every other day for 10 days (5 doses) 5, 6, 7
Oseltamivir is not recommended for patients with end-stage renal disease not undergoing dialysis 1.
Critical Timing Considerations
Treatment must be initiated within 48 hours of symptom onset for maximum benefit 7, 1, 2. Earlier initiation provides superior outcomes: treatment within 12 hours reduces illness duration by an additional 74.6 hours compared to treatment at 48 hours, while treatment within 24 hours provides an additional 53.9 hours of benefit 2.
Formulations and Administration
Oseltamivir is available as:
- Capsules: 30 mg, 45 mg, and 75 mg 5, 7, 1
- Oral suspension: 6 mg/mL when reconstituted from powder 5, 7, 1
Administration with food improves gastrointestinal tolerability, though the drug can be taken with or without meals 5, 7, 1. For the oral suspension: 30 mg = 5 mL, 45 mg = 7.5 mL, 60 mg = 10 mL, 75 mg = 12.5 mL 5, 6.
Common Adverse Effects
Nausea and vomiting are the most common adverse events, occurring in approximately 10% of patients 3, 2, 8. These gastrointestinal effects are typically mild, transient, and resolve within 1-2 days 2. Taking oseltamivir with food significantly reduces these symptoms 5, 2. Other adverse events include headache, diarrhea, and skin reactions 6.
Clinical Efficacy
Oseltamivir reduces the duration of influenza illness by 1-1.5 days and decreases illness severity by up to 38% when initiated within 36 hours of symptom onset 3. The drug also significantly reduces secondary complications including otitis media, bronchitis, pneumonia, and sinusitis, thereby decreasing antibiotic use 3, 2. For prophylaxis, oseltamivir prevents naturally acquired influenza by >70% in unvaccinated adults and demonstrates 92% protective efficacy in previously vaccinated high-risk elderly patients 3.
Important Drug Interactions
Avoid live attenuated influenza vaccine (LAIV) within 48 hours before oseltamivir use, and do not use oseltamivir for 14 days after LAIV vaccination 6. The potential for other drug interactions is low due to oseltamivir's predictable pharmacokinetic profile 4.