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Here is a complete overview of Oseltamivir (Tamiflu) - its uses, mechanism, dosing, and important clinical notes:
Oseltamivir (Tamiflu) - Uses & Overview
Mechanism of Action
Oseltamivir is an oral prodrug (oseltamivir phosphate) that is cleaved by esterases in the liver and GI tract to its active form, oseltamivir carboxylate - a transition-state analogue of sialic acid. It selectively inhibits the neuraminidase enzyme of influenza A and B viruses.
Neuraminidase cleaves sialic acid residues on the cell surface, allowing newly formed virions to be released. By inhibiting this enzyme, oseltamivir causes viral aggregation at the cell surface and prevents spread within the respiratory tract.
- Goodman & Gilman's Pharmacological Basis of Therapeutics
Approved Clinical Uses
1. Treatment of Influenza A and B
- Approved for acute, uncomplicated influenza in adults and children (including neonates >2 weeks old)
- Must be started within 48 hours of symptom onset for best effect
- Reduces illness duration by 1-2 days in healthy adults and children
- Reduces risk of complications (e.g., bronchitis, pneumonia) requiring antibiotics by 40-50%
- Reduces hospitalization risk by about 50% in adults
- For hospitalized patients or those with severe/progressive disease, oseltamivir should be started regardless of how long symptoms have been present
2. Prophylaxis (Prevention) of Influenza
- Approved for prevention of influenza A and B in persons aged 1 year and older
- Dose: 75 mg once daily during exposure period
- Used for post-exposure prophylaxis (e.g., household contacts of confirmed influenza cases) and seasonal prophylaxis in high-risk individuals
- When used during a typical influenza season, reduces risk of influenza illness by ~90% in household contacts
3. High-Risk and Hospitalized Patients
Oseltamivir is the drug of choice for:
- Patients hospitalized with severe influenza
- Elderly patients (especially nursing home residents)
- Patients with chronic medical conditions (cardiopulmonary disease, diabetes, immunosuppression)
- Pregnant women
- Immunocompromised patients (infectious disease specialist consultation recommended)
- Harrison's Principles of Internal Medicine 22E (2025)
4. Pandemic and Novel Influenza Strains
- Used during the 2009 H1N1 pandemic (swine flu); novel H1N1 remained susceptible to oseltamivir
- Pandemic influenza A (H1N1) strains are susceptible, while older seasonal H1N1 strains became virtually 100% resistant worldwide
- Park's Textbook of Preventive and Social Medicine
Dosing (Adults)
| Indication | Dose |
|---|---|
| Treatment | 75 mg twice daily x 5 days |
| Prophylaxis | 75 mg once daily |
Dose must be adjusted in renal impairment (drug is excreted by the kidneys via tubular secretion).
Pharmacokinetics
- Oral bioavailability is high; food does not reduce absorption but decreases GI side effects
- Achieves good levels in bronchoalveolar lavage, middle ear fluid, and sinuses
- Probenecid doubles the plasma half-life (blocks tubular secretion) - a notable drug interaction
Side Effects
- GI: Nausea, vomiting, abdominal pain (most common; reduced by taking with food)
- CNS: Neuropsychiatric events (confusion, abnormal behavior) reported, especially in children - though it is debated whether this is drug-related or due to influenza itself
Resistance
- Resistance can arise from mutations in viral neuraminidase or hemagglutinin
- Resistance rate: ~15% in healthy children, ~1% in adults
- More common in immunocompromised patients
- Resistant virus can transmit person-to-person
- Harrison's Principles of Internal Medicine 22E
Important Limitations
- Not a substitute for the annual flu vaccine
- Less active against influenza B than influenza A
- Does not work against bacterial infections
- Should not be delayed pending lab confirmation in high-risk or hospitalized patients
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