## First-Line Antiviral for Influenza A and B **Key Point:** Oseltamivir (Tamiflu) is the gold-standard first-line neuraminidase inhibitor for treatment of both influenza A and B, especially when started within 48 hours of symptom onset. ### Mechanism & Efficacy Oseltamivir is a prodrug that inhibits viral neuraminidase, preventing release of progeny virions from infected cells. It reduces symptom duration by approximately 1 day and decreases risk of complications when given early. ### Dosing & Administration - **Standard dose:** 75 mg orally twice daily for 5 days - **Renal adjustment:** Required if CrCl < 30 mL/min - **Timing:** Most effective if started within 48 hours of symptom onset ### Comparison with Other Antivirals | Agent | Class | Coverage | Route | Resistance | Notes | |-------|-------|----------|-------|-----------|-------| | **Oseltamivir** | Neuraminidase inhibitor | A + B | Oral | Rare (~1%) | First-line; excellent bioavailability | | Zanamivir | Neuraminidase inhibitor | A + B | Inhaled | Rare | Alternative; risk of bronchospasm in asthma/COPD | | Amantadine | M2 channel blocker | A only | Oral | Common (>50%) | Obsolete; poor efficacy due to resistance | | Rimantadine | M2 channel blocker | A only | Oral | Common (>50%) | Obsolete; not recommended | **High-Yield:** Neuraminidase inhibitors (oseltamivir, zanamivir) are preferred over M2 blockers because: 1. Cover both influenza A and B 2. Minimal resistance globally 3. Better clinical efficacy 4. M2 blockers are no longer recommended due to widespread resistance **Clinical Pearl:** Even though oseltamivir is most effective within 48 hours, treatment may still be considered in hospitalized patients or those with severe illness beyond 48 hours. **Warning:** Do not confuse oseltamivir with amantadine—amantadine has >50% resistance rates and is no longer recommended for influenza treatment. [cite:Harrison 21e Ch 195]
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