Amoxicillin 125 mg Twice Daily is Severely Underdosed for an 18 kg Child
This dosing regimen provides only 13.9 mg/kg/day, which is grossly inadequate and should be corrected immediately to prevent treatment failure and potential complications.
Weight-Based Dosing Calculation
For an 18 kg child receiving 125 mg twice daily (250 mg/day total):
- Current dose: 250 mg/day ÷ 18 kg = 13.9 mg/kg/day 1
- This is less than one-third of the minimum recommended dose for any pediatric infection 1, 2
Correct Dosing Based on Indication
For Mild to Moderate Respiratory Infections
- Standard dose: 45 mg/kg/day divided into 2 doses 1, 2
- For 18 kg child: 45 mg/kg/day × 18 kg = 810 mg/day total
- Practical dosing: 400 mg twice daily (approximately 44 mg/kg/day) 1
- This can be achieved with 8 mL of 250 mg/5 mL suspension twice daily 1
For Severe Infections or High Pneumococcal Resistance Areas
- High-dose regimen: 90 mg/kg/day divided into 2 doses 1, 2
- For 18 kg child: 90 mg/kg/day × 18 kg = 1,620 mg/day total
- Practical dosing: 800 mg twice daily (approximately 89 mg/kg/day) 1, 2
- This requires 16 mL of 250 mg/5 mL suspension twice daily 1
High-dose therapy is specifically indicated for 1, 2:
- Community-acquired pneumonia
- Children <2 years old
- Recent antibiotic exposure within past 30 days
- Daycare attendance
- Areas with >10% penicillin-resistant S. pneumoniae
For Group A Streptococcal Infections
- Recommended dose: 50-75 mg/kg/day divided into 2 doses for 10 days 1
- For 18 kg child: 900-1,350 mg/day total
- Practical dosing: 450-675 mg twice daily 1
FDA-Approved Dosing Guidelines
The FDA label confirms that for children ≥3 months and <40 kg 3:
- Mild/moderate infections: 25 mg/kg/day divided every 12 hours (minimum 450 mg/day for 18 kg child)
- Severe infections: 45 mg/kg/day divided every 12 hours (810 mg/day for 18 kg child)
- The current 125 mg twice daily dose falls far below even the lowest FDA-approved threshold 3
Clinical Implications of Underdosing
Treatment failure is highly likely with 13.9 mg/kg/day because 1, 4:
- Inadequate serum concentrations to maintain levels above MIC for susceptible organisms
- Increased risk of bacterial persistence and relapse
- Potential development of antibiotic resistance
- No clinical improvement expected within the standard 48-72 hour window 1
Research demonstrates that even 50 mg/kg/day divided twice daily achieves equivalent efficacy to three-times-daily dosing for non-severe pneumonia 4, but 13.9 mg/kg/day has no evidence base for any indication.
Recommended Correction
Immediately increase the dose based on the suspected or confirmed infection 1, 2:
If infection type unknown or mild-moderate respiratory infection: Prescribe 400 mg (8 mL of 250 mg/5 mL suspension) twice daily 1
If severe infection, pneumonia, or high-resistance risk factors present: Prescribe 800 mg (16 mL of 250 mg/5 mL suspension) twice daily 1, 2
Ensure treatment duration of 7-10 days for most infections, with pneumonia specifically requiring 10 days 1
Monitor for clinical improvement within 48-72 hours; if no improvement occurs, reevaluation is mandatory 1, 2
Critical Pitfall to Avoid
The most common dosing error in pediatric amoxicillin prescribing is using adult fixed doses (e.g., 250 mg) rather than weight-based calculations 1. Always calculate mg/kg/day first, then select the appropriate suspension volume or tablet strength to achieve the target dose 1, 3.