Amoxicillin Dosing for an 18.5 kg Child
For a child weighing 18.5 kg, prescribe amoxicillin 90 mg/kg/day divided into 2 doses (approximately 830 mg twice daily) for most common infections, or 45 mg/kg/day divided into 2 doses (approximately 415 mg twice daily) for mild infections. 1
Dosing Algorithm by Infection Severity
The FDA-approved dosing for pediatric patients weighing less than 40 kg follows a clear severity-based approach 1:
For Mild to Moderate Infections
(Ear/Nose/Throat, Skin/Skin Structure, Genitourinary Tract)
- 25 mg/kg/day divided every 12 hours (approximately 230 mg twice daily for 18.5 kg child)
- Alternative: 20 mg/kg/day divided every 8 hours
For Severe Infections or Lower Respiratory Tract Infections
- 45 mg/kg/day divided every 12 hours (approximately 415 mg twice daily for 18.5 kg child)
- Alternative: 40 mg/kg/day divided every 8 hours
For Pneumonia (Community-Acquired)
The most recent IDSA/PIDS guidelines recommend 90 mg/kg/day in 2 divided doses (approximately 830 mg twice daily) for Streptococcus pneumoniae with penicillin MICs <2.0 µg/mL 2. This high-dose regimen addresses increasing pneumococcal resistance and is the preferred first-line therapy.
For non-severe pneumonia in resource-limited settings, 50 mg/kg in two divided doses for 5 days is acceptable 3.
Practical Dosing Considerations
Key calculation for this 18.5 kg child:
- Low-dose: 25 mg/kg/day = 462.5 mg/day (230 mg BID)
- Standard-dose: 45 mg/kg/day = 832.5 mg/day (415 mg BID)
- High-dose: 90 mg/kg/day = 1,665 mg/day (830 mg BID)
Important Caveats
Weight-based dosing is critical: The FDA label explicitly provides weight-based dosing for children under 40 kg, not age-based dosing 1. Recent evidence shows that larger children are systematically underdosed when age-based dosing is used 4.
Duration matters: Recent high-quality evidence from the CAP-IT trial (2021) demonstrated that 3-day courses are non-inferior to 7-day courses for community-acquired pneumonia 5, 6. However, for Streptococcus pyogenes infections, maintain 10-day treatment to prevent acute rheumatic fever 1.
Administration timing: Give amoxicillin at the start of meals to minimize gastrointestinal intolerance 1.
Suspension preparation: For oral suspension, ensure proper reconstitution and shake well before each dose. Refrigeration is preferable but not required; discard after 14 days 1.
Common Pitfalls to Avoid
Don't cap at "adult doses" prematurely: Children weighing 18.5 kg requiring high-dose amoxicillin (90 mg/kg/day = 1,665 mg/day) should receive this full dose, even though it exceeds typical adult dosing of 1,500 mg/day. Evidence shows clinicians inappropriately limit doses in heavier children 4, 7.
Don't use age-based dosing: Weight-based calculations are more accurate and prevent systematic under-dosing 8.
Specify complete prescription details: Include the daily dose, number of divided doses, duration, indication, and the child's weight on the prescription to prevent pharmacy confusion 8.