What is the recommended prophylactic dosing regimen of oseltamivir (Tamiflu) after exposure to influenza, including adult/adolescent dose, weight‑based pediatric dose, timing, duration, renal‑impairment adjustments, contraindications, and the alternative of inhaled zanamivir?

Tamiflu Prophylactic Dosing

For post-exposure influenza prophylaxis, adults and adolescents ≥13 years should receive oseltamivir 75 mg orally once daily for 10 days, initiated within 48 hours of exposure to an infected individual. 1, 2

Adult and Adolescent Prophylaxis (≥13 years)

• The standard prophylactic dose is 75 mg orally once daily for 10 days following household or close contact exposure to a confirmed or suspected influenza case. 1, 2
• Prophylaxis should be initiated within 48 hours of exposure for maximum effectiveness, though it may be considered up to 10 days post-exposure in certain circumstances. 2
• For seasonal or community outbreak prophylaxis, oseltamivir 75 mg once daily may be continued for up to 6 weeks during periods of local influenza activity. 2, 3
• Oseltamivir can be taken with or without food, though administration with meals significantly reduces gastrointestinal side effects (nausea occurs in ~12% vs. 7% with placebo). 1, 4

Pediatric Prophylaxis (≥1 year)

Weight-based dosing is used for children ≥12 months, administered once daily for 10 days: 1

• ≤15 kg (≤33 lb): 30 mg once daily 1
• >15–23 kg (>33–51 lb): 45 mg once daily 1
• >23–40 kg (>51–88 lb): 60 mg once daily 1
• >40 kg (>88 lb): 75 mg once daily 1

The oral suspension formulation (6 mg/mL) provides the following volumes: 30 mg = 5 mL, 45 mg = 7.5 mL, 60 mg = 10 mL, 75 mg = 12.5 mL. 1, 5

Infant Prophylaxis (<1 year)

• Infants 3–11 months: 3 mg/kg per dose once daily for 10 days (if prophylaxis is deemed necessary). 1, 5, 2
• Infants <3 months: Prophylaxis is not recommended unless the clinical situation is judged critical, due to limited safety and efficacy data in this age group. 1, 2
• Preterm infants: Prophylaxis dosing is not established; consult pediatric infectious disease if prophylaxis is considered essential. 1

Renal Impairment Adjustments

For patients with creatinine clearance 10–30 mL/min, dose reduction is mandatory: 1, 6

• Option 1: 30 mg once daily for 10 days 1, 6, 2
• Option 2: 75 mg every other day for 10 days (total of 5 doses) 1, 6, 2

For patients with creatinine clearance <10 mL/min or on hemodialysis: Oseltamivir is not recommended due to insufficient data. 6

Age alone does not require dose adjustment in elderly patients (≥65 years), but renal function must be assessed as creatinine clearance declines with aging. 5

Alternative: Inhaled Zanamivir

For patients who cannot tolerate oseltamivir or have contraindications, zanamivir is an alternative: 1

• Adults and children ≥5 years: 10 mg (two 5-mg inhalations) once daily for 10 days 1
• Zanamivir is contraindicated in patients with underlying airway disease (asthma, COPD) due to risk of bronchospasm. 1

Contraindications and Precautions

• Hypersensitivity to oseltamivir or any component of the formulation is the only absolute contraindication. 6
• Pregnancy and breastfeeding are not contraindications; the same adult dosing applies. 5
• Live attenuated influenza vaccine (LAIV): Avoid oseltamivir for 2 weeks before and 48 hours after LAIV administration, as the antiviral may interfere with vaccine efficacy. 6, 2
• Egg allergy is not a contraindication to oseltamivir use. 1

Critical Timing Considerations

• Initiate prophylaxis within 48 hours of exposure to an infected individual for optimal protection (74–87% protective efficacy). 2, 3
• Earlier initiation (within 24 hours) may provide even greater benefit. 4
• Prophylaxis started >48 hours post-exposure has reduced effectiveness but may still be considered in high-risk individuals or outbreak settings. 2

Common Pitfalls and Caveats

Do not confuse prophylaxis dosing (once daily) with treatment dosing (twice daily) – this is the most frequent prescribing error and can lead to either underdosing prophylaxis or overdosing treatment. 2

Do not use weight-based categorical dosing (≤15 kg = 30 mg) for infants <12 months – infants require mg/kg dosing (3 mg/kg or 3.5 mg/kg depending on age). 5

Do not apply term infant dosing to preterm infants – preterm infants require lower postmenstrual age-based dosing to avoid toxic concentrations due to immature renal function. 5, 6

Do not round doses inappropriately in infants – calculate the exact mg/kg dose and measure the corresponding volume precisely using a calibrated oral syringe (not household spoons). 5

Do not forget renal dose adjustments – failure to reduce the dose in patients with creatinine clearance <60 mL/min can lead to drug accumulation and toxicity. 1, 6

Do not stop prophylaxis early – complete the full 10-day course even if no symptoms develop, to ensure adequate viral suppression and prevent breakthrough infection. 2

Formulation and Administration Details

• Capsules: Available in 30 mg, 45 mg, and 75 mg strengths. 1, 5
• Oral suspension: Reconstituted to 6 mg/mL concentration; if commercial suspension is unavailable, pharmacies can compound it according to package insert instructions. 1, 5
• Administration with food reduces nausea and vomiting (most common adverse effects, occurring in 10–15% of patients). 1, 4
• Gastrointestinal side effects are typically mild, transient, and resolve within 1–2 days; only ~1% of patients discontinue therapy due to these symptoms. 4