What dose and dosing frequency of amoxicillin should be given to a 4‑year‑old child weighing approximately 18 kg?

Amoxicillin Dosing for a 4-Year-Old Child (18 kg)

For a 4-year-old child weighing 18 kg, prescribe amoxicillin 90 mg/kg/day divided into 2 doses (810 mg/day total, or approximately 400 mg twice daily) for most respiratory infections requiring antibiotic therapy. This high-dose regimen is the preferred first-line treatment for presumed bacterial pneumonia and other common respiratory infections in this age group 1, 2, 3.

Dosing Algorithm by Clinical Scenario

For Presumed Bacterial Pneumonia (Most Common Indication)

• Standard recommendation: 90 mg/kg/day in 2 divided doses
  • For this 18 kg child: 810 mg/day (approximately 400 mg every 12 hours)
  • Maximum single dose: 2000 mg 2, 3
  • Duration: 5-10 days (minimum 5 days for uncomplicated cases) 2, 4

Rationale: High-dose amoxicillin (80-90 mg/kg/day) is recommended for children ≥5 years old with presumed bacterial community-acquired pneumonia to overcome potential pneumococcal resistance 1, 2. The twice-daily dosing improves compliance while maintaining efficacy 5, 6.

For Acute Otitis Media or Sinusitis

• Mild/moderate cases: 45 mg/kg/day in 2 doses (810 mg/day for this child)
• Severe cases or recent antibiotic exposure: 90 mg/kg/day in 2 doses 7, 3

For Group A Streptococcal Infections

• Dose: 50-75 mg/kg/day in 2 doses
  • For this 18 kg child: 900-1350 mg/day (approximately 450-675 mg every 12 hours)
  • Duration: Minimum 10 days to prevent rheumatic fever 1, 2, 3

Practical Prescribing Details

What to write on the prescription:

• Drug: Amoxicillin oral suspension
• Dose: 400 mg (or 8 mL of 250 mg/5 mL suspension)
• Frequency: Every 12 hours (twice daily)
• Duration: 5-7 days for pneumonia; 10 days for strep throat
• Child's weight: 18 kg
• Indication: [specify diagnosis] 8

Key Clinical Considerations

Dosing Frequency

Twice-daily dosing is equivalent to three-times-daily dosing for the same total daily dose and significantly improves adherence 5, 6. The evidence shows no difference in treatment failure rates between 50 mg/kg/day given twice versus three times daily 5.

Duration of Therapy

• For uncomplicated pneumonia: 3 days may be sufficient, though 5-7 days shows slightly faster symptom resolution 4
• For streptococcal pharyngitis: Full 10 days required 3
• Treatment should continue 48-72 hours beyond symptom resolution 3

Common Pitfalls to Avoid

1. Underdosing larger children: There is documented disparity where children >15 kg often receive inadequate mg/kg dosing because prescribers hesitate to exceed "adult maximum doses" 9. For this 18 kg child, the calculated 810 mg/day is appropriate and should not be reduced.

2. Using age-based rather than weight-based dosing: Always calculate based on actual weight 8

3. Inadequate duration for streptococcal infections: Must complete full 10-day course to prevent rheumatic fever 3

4. Not specifying indication on prescription: This leads to unnecessary pharmacist-prescriber consultations 8

When to Use Higher Doses (90 mg/kg/day)

• Pneumonia (any severity) 1, 2
• Age <2 years 7
• Recent antibiotic exposure (within 30 days) 7
• Daycare attendance 7
• Areas with high pneumococcal resistance (>10% non-susceptible strains) 7
• Moderate to severe illness 7

Administration Tips

• Give at the start of meals to minimize gastrointestinal intolerance 3
• Shake suspension well before each dose 3
• Can mix with formula, milk, juice, or water if needed; must be taken immediately after mixing 3
• Refrigeration preferred but not required; discard after 14 days 3

Formulation Preference

Use dispersible tablets (250 mg or 500 mg) when available rather than suspension, as they are the WHO-recommended formulation for children and allow more precise dosing 6. For this 18 kg child receiving 400 mg twice daily, one and a half 250 mg dispersible tablets would provide accurate dosing.