TAMIFLU
Clinical safety rating: caution
Comprehensive clinical and safety monograph for TAMIFLU (TAMIFLU).
Oseltamivir phosphate is a prodrug that is hydrolyzed to the active metabolite oseltamivir carboxylate, a selective inhibitor of influenza virus neuraminidase, an enzyme required for viral replication and release from infected cells.
| Metabolism | Oseltamivir is extensively converted to oseltamivir carboxylate by hepatic esterases; neither the prodrug nor active metabolite is a substrate for or inhibitor of CYP450 enzymes. |
| Excretion | Renal excretion of the active metabolite oseltamivir carboxylate accounts for >90% of absorbed drug via glomerular filtration and tubular secretion; <1% excreted as parent oseltamivir in urine; fecal elimination <5%. |
| Half-life | Terminal elimination half-life of oseltamivir carboxylate is 4.4 hours (range 3.9–5.0 h) in healthy adults, allowing twice-daily dosing; prolonged to 18–24 hours in severe renal impairment (CrCl <10 mL/min). |
| Protein binding | Parent oseltamivir: 45% bound to human serum albumin; oseltamivir carboxylate: <3% bound (negligible). |
| Volume of Distribution | Apparent volume of distribution (Vd/F) for oseltamivir carboxylate: 0.6–0.8 L/kg, indicating distribution throughout total body water, with penetration into respiratory tract, middle ear, and CSF. |
| Bioavailability | Oral bioavailability of oseltamivir as prodrug is 80% (range 70–85%) due to first-pass hepatic hydrolysis to carboxylate; absorption unaffected by food. |
| Onset of Action | Oral administration: reduction in viral shedding and symptom relief begins within 12 hours of first dose; prophylactic efficacy observed within 2 weeks of daily dosing. |
| Duration of Action | Clinical effect persists for 24 hours post-dose, supporting twice-daily dosing for 5 days (treatment) or 10 days (prophylaxis). Maximum antiviral effect occurs within 48 hours of initiation when started within 48 hours of symptom onset. |
75 mg orally twice daily for 5 days
| Dosage form | FOR SUSPENSION |
| Renal impairment | CrCl 30-60 mL/min: 30 mg twice daily; CrCl 10-29 mL/min: 30 mg once daily; CrCl <10 mL/min: not recommended |
| Liver impairment | No dose adjustment required for mild to moderate hepatic impairment; no data for severe impairment |
| Pediatric use | Weight ≤15 kg: 30 mg twice daily; 15.1-23 kg: 45 mg twice daily; 23.1-40 kg: 60 mg twice daily; >40 kg: 75 mg twice daily for 5 days |
| Geriatric use | No specific dose adjustment; use standard adult dosing with attention to renal function |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Comprehensive clinical and safety monograph for TAMIFLU (TAMIFLU).
| Breastfeeding | Excreted into breast milk in small amounts; M/P ratio not established. Infant exposure estimated <1% of maternal dose due to low oral bioavailability. Considered compatible with breastfeeding; monitor infant for diarrhea or rash. |
| Teratogenic Risk | FDA Pregnancy Category C. Animal studies (rats and rabbits) showed no teratogenicity at doses up to 13 times human exposure. Limited human data: no increased risk of major malformations reported in first trimester exposure. Potential risks in second/third trimester unknown; use only if benefit outweighs risk. |
■ FDA Black Box Warning
None.
| Serious Effects |
Known hypersensitivity (e.g., anaphylaxis, serious skin reactions) to oseltamivir or any component of the product.
| Precautions | Serious skin/hypersensitivity reactions (e.g., Stevens-Johnson syndrome, toxic epidermal necrolysis), neuropsychiatric events (delirium, hallucinations, abnormal behavior, self-injury) reported mainly in pediatric patients, risk of bacterial superinfection, renal impairment requires dose adjustment, efficacy not established in patients with severe underlying respiratory disease or immunocompromised status. |
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| Fetal Monitoring |
| No specific fetal monitoring required. Monitor for maternal adverse effects: nausea, vomiting, headache, neuropsychiatric events. For severe influenza, monitor respiratory status and fetal well-being if indicated. |
| Fertility Effects | No known negative impact on human fertility. Animal studies at high doses showed no adverse effects on fertility or reproductive performance. |