OSELTAMIVIR PHOSPHATE
Clinical safety rating: safe
No significant drug interactions May cause nausea and vomiting and rare neuropsychiatric events.
Oseltamivir phosphate is a neuraminidase inhibitor. It is a prodrug that is hydrolyzed to the active metabolite oseltamivir carboxylate, which inhibits influenza A and B neuraminidases, preventing viral replication by blocking the cleavage of sialic acid residues and release of progeny virions.
| Metabolism | Oseltamivir phosphate is extensively converted to oseltamivir carboxylate by hepatic esterases. Neither the parent nor the active metabolite is a substrate for or inhibitor of cytochrome P450 enzymes. |
| Excretion | Renal: 60-70% of total clearance as oseltamivir carboxylate via glomerular filtration and tubular secretion; unchanged oseltamivir <5%. Fecal: <20% as oseltamivir carboxylate. Biliary: negligible. |
| Half-life | Oseltamivir carboxylate: 6-10 hours (terminal) in adults with normal renal function; prolonged to >20 hours in severe renal impairment (CrCl <10 mL/min). |
| Protein binding | Oseltamivir: ~42% bound to plasma proteins (albumin and alpha-1 acid glycoprotein). Oseltamivir carboxylate: <3% bound. |
| Volume of Distribution | Oseltamivir: 0.6-0.7 L/kg (central compartment); oseltamivir carboxylate: 0.3 L/kg. Indicates extensive tissue penetration, including lungs and nasal mucosa. |
| Bioavailability | Oral (prodrug): 75% bioavailability as oseltamivir carboxylate after first-pass hepatic conversion by esterases; food does not significantly affect. |
| Onset of Action | Oral: Clinical benefit begins within 24-48 hours of initiation if started within 48 hours of symptom onset; maximal antiviral effect at steady state (~3 days). |
| Duration of Action | Oral: Duration of therapy typically 5 days; dose adjustment for renal impairment prolongs drug presence. Clinical benefit persists while drug concentrations exceed viral IC90. |
75 mg orally twice daily for 5 days.
| Dosage form | CAPSULE |
| Renal impairment | Creatinine clearance (CrCl) >60 mL/min: no adjustment; CrCl 30-60 mL/min: 75 mg once daily for 5 days; CrCl 10-30 mL/min: 75 mg once daily for 5 days; CrCl <10 mL/min: not recommended (use only for pandemic influenza if benefit outweighs risk, with reduced dose per expert guidance). |
| Liver impairment | No dosage adjustment required for mild to moderate hepatic impairment (Child-Pugh class A or B); no data available for severe hepatic impairment (Child-Pugh class C). |
| Pediatric use | Children ≥1 year and ≤40 kg: weight-based dosing for 5 days: ≤15 kg: 30 mg twice daily; >15 kg to 23 kg: 45 mg twice daily; >23 kg to 40 kg: 60 mg twice daily; >40 kg: 75 mg twice daily. Infants 2 weeks to <1 year: 3 mg/kg/dose twice daily. Infants <2 weeks: 3 mg/kg/dose once daily. |
| Geriatric use | No specific dose adjustment required for elderly patients with normal renal function; however, assess renal function and adjust dose accordingly due to age-related decline in CrCl. |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
No significant drug interactions May cause nausea and vomiting and rare neuropsychiatric events.
| FDA category | Animal |
| Breastfeeding | Oseltamivir and its active metabolite are excreted into human milk in low concentrations. M/P ratio: 0.014 for oseltamivir, 0.038 for oseltamivir carboxylate. Infant exposure via milk is minimal (<1% of maternal weight-adjusted dose). Considered compatible with breastfeeding. |
| Teratogenic Risk | FDA Pregnancy Category C. Animal studies showed no teratogenicity at clinically relevant doses; limited human data. First trimester: no increased risk of major malformations in cohort studies. Second and third trimesters: no specific fetal risks identified. Use only if benefit outweighs risk. |
■ FDA Black Box Warning
None.
| Common Effects | Nausea |
| Serious Effects |
["Hypersensitivity to oseltamivir or any component of the formulation","Severe renal impairment (CrCl <10 mL/min) not on dialysis"]
| Precautions | ["Risk of neuropsychiatric events (e.g., delirium, self-injury) mainly reported in pediatric patients, especially during treatment; monitor for abnormal behavior","Serious skin/hypersensitivity reactions (e.g., Stevens-Johnson syndrome, toxic epidermal necrolysis); discontinue if allergic reaction occurs","No evidence of efficacy against non-influenza viral infections or bacterial superinfections","Not a substitute for annual influenza vaccination","Use with caution in patients with renal impairment; dose adjustment required for CrCl 30-60 mL/min","Pregnancy: limited data; use only if clearly needed","Lactation: caution; excreted in breast milk in animal studies"] |
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| Fetal Monitoring | No specific fetal monitoring required. Monitor maternal clinical status for influenza resolution and adverse effects (e.g., nausea, vomiting). In neonates exposed in utero, observe for potential antiviral effects; no standard monitoring protocol. |
| Fertility Effects | No evidence of impaired fertility in animal studies at clinically relevant doses. No human data on fertility impact. Reversible reduction in sperm motility and count reported in male rats at high doses, not confirmed in humans. |