What is the appropriate oseltamivir (Tamiflu) dosage for a healthy 21‑year‑old adult with normal renal function for treatment and for post‑exposure prophylaxis?

Oseltamivir Dosing for a 21-Year-Old Adult

For a healthy 21-year-old with normal renal function, the standard oseltamivir dose is 75 mg orally twice daily for 5 days for treatment, or 75 mg once daily for 10 days for post-exposure prophylaxis. 1, 2, 3

Treatment Regimen

• Dose: 75 mg orally twice daily (every 12 hours) 1, 4
• Duration: 5 days, regardless of symptom improvement 2, 4
• Timing: Initiate within 48 hours of symptom onset for maximum benefit; earlier initiation (within 12–24 hours) provides the greatest reduction in illness duration 2, 5
• Administration: Can be taken with or without food, though taking with meals significantly reduces nausea and vomiting (which occur in ~10% of patients) 1, 2, 4

Post-Exposure Prophylaxis Regimen

• Dose: 75 mg orally once daily 1, 3
• Duration: 10 days after close contact with an infected individual 2, 3
• Timing: Start within 48 hours of exposure 2, 3

Renal Function Considerations

• Normal renal function (CrCl ≥30 mL/min): No dose adjustment needed 1, 4
• Moderate-to-severe impairment (CrCl 10–30 mL/min): Reduce treatment dose to 75 mg once daily for 5 days; prophylaxis dose to 30 mg once daily or 75 mg every other day for 10 days 1
• Age alone does not require dose adjustment: A 21-year-old with normal renal function receives standard adult dosing 1

Available Formulations

• Capsules: 30 mg, 45 mg, and 75 mg strengths 1, 2
• Oral suspension: 6 mg/mL when reconstituted (12.5 mL = 75 mg dose) 1, 2

Critical Timing and Efficacy

• Treatment within 12 hours of symptom onset reduces illness duration by an additional 74.6 hours compared to starting at 48 hours 5
• Treatment within 24 hours reduces illness duration by an additional 53.9 hours compared to starting at 48 hours 5
• Complete the full 5-day course even if symptoms resolve earlier to ensure adequate viral suppression and prevent resistance 2, 4

Common Adverse Effects and Management

• Nausea (≈10%) and vomiting (≈9%) are the most common side effects 2, 5, 6
• These effects are mild, transient (resolve within 1–2 days), and significantly reduced when taken with food 2, 5, 6
• Only ~1% of patients discontinue therapy due to gastrointestinal effects 2

Important Caveats

• Do not confuse treatment dosing (twice daily) with prophylaxis dosing (once daily) 3
• Avoid live attenuated influenza vaccine (LAIV) within 48 hours before oseltamivir and for 14 days after LAIV 2
• Oseltamivir is effective against both influenza A and B strains 4, 7