Generic Name

Oseltamivir

Mechanism

Oseltamivir is a *neuraminidase inhibitor* that blocks the enzyme neuraminidase on the surface of influenza A and B viruses. This inhibition:
• Prevents cleavage of sialic acid residues,
• Reduces release of newly formed virions from infected cells,
• Limits spread of the virus within the respiratory tract.

The agent thus shortens disease duration and mitigates symptom severity when initiated early.

Pharmacokinetics

• Absorption – Rapid oral absorption; peak plasma concentration occurs 0.7–1 h post‑dose.
• Bioavailability – 15 % of the administered drug is converted into the active metabolite, oseltamivir carboxylate.
• Distribution – Volume of distribution ≈ 3 L/kg; low protein binding (< 10 %).
• Metabolism – Minimal hepatic metabolism; primarily hydrolyzed to the active carboxylate form.
• Elimination – Renal excretion unchanged (≈ 70 %) and as carboxylate (≈ 30 %).
• Half‑life – ~ 10–17 h for the active metabolite; 2–3 h for the parent compound.
• Special Populations
• Renal impairment – Requires dose reduction in CrCl < 30 mL/min.
• Hepatic impairment – No dose adjustment needed.
• Pregnancy – Category C; used when benefits outweigh risks.

Indications

• Treatment of confirmed or suspected influenza A or B in adults and children, with initiation ideally within 48 h of symptom onset.
• Prevention of influenza in individuals exposed to infected persons, including household contacts and healthcare workers.

Contraindications

• Known hypersensitivity to oseltamivir or any excipients.
• Severe renal dysfunction (CrCl < 10 mL/min) – contraindicated.
• Pregnancy – Use only if clearly needed; fetal monitoring advised.
• Seizure disorders – Elevated seizure risk, particularly at high doses.
• Children < 2 weeks – Not recommended due to limited data.

Dosing

• Administer orally with water; can be taken with or without food.
• For patients unable to swallow, the liquid formulation can be used.

Adverse Effects

• Common
• Nausea, vomiting, and abdominal discomfort (≈ 10‑15 %)
• Dizziness, headache, and fatigue (≈ 5‑10 %)
• Serious
• Seizures – ~ 1 per 10 000, more likely at high doses or in renal disease.
• Neuropsychiatric events – agitation, confusion, hallucinations (rare).
• Allergic reactions – rash, urticaria, anaphylaxis (rare).

Monitoring

• Renal function – Serum creatinine and eGFR before initiation and at trough if compromised.
• Neurologic status – Watch for new or worsening seizures or altered mental status.
• Signs of hypersensitivity – Monitor for rash, pruritus, angioedema.
• Influenza resolution – Document symptom improvement and fever clearance.

Clinical Pearls

• Start early – Administration within 48 h of symptom onset maximizes benefit; delaying beyond 48 h yields minimal advantage.
• Dose adjust renal – In patients with CrCl 30‑50 mL/min, halve the dose to 30 mg QD to avoid drug accumulation.
• High‑dose for immunocompromised – 75 mg QD for up to 10 days can prevent viral resistance and prolonged shedding in transplant recipients.
• Use in pregnancy safely – Data support its safety; counsel patients on the low teratogenic risk versus substantial benefit during influenza outbreaks.
• Overdose management – No specific antidote; supportive care and repeat dosing according to guidelines reduce systemic exposure.
• Drug interactions – Minimal; avoid concurrent use with theophylline or cholinergic agents in patients prone to seizures.
• Patient education – Emphasize the importance of completing the full course even if symptoms improve, to prevent resistance.

*Keywords: Oseltamivir, antiviral, neuraminidase inhibitor, influenza treatment, pharmacokinetics, renal dosing, neonatal prophylaxis, seizure risk.*