Amoxicillin Dosing for an 11-Year-Old Child Weighing 85 Pounds (38.6 kg)
For most common pediatric infections in this child, prescribe amoxicillin 45 mg/kg/day divided into two doses (approximately 870 mg twice daily), which can be administered as 10.9 mL of the 400 mg/5 mL suspension per dose. 1
Standard-Dose Regimen (Mild-to-Moderate Infections)
For uncomplicated ear/nose/throat infections, skin infections, or genitourinary tract infections without risk factors for resistant organisms, the FDA-approved standard dose is 45 mg/kg/day divided every 12 hours (approximately 870 mg twice daily for this 38.6 kg child). 11
This translates to approximately 10.9 mL of the 400 mg/5 mL suspension twice daily or approximately 17.4 mL of the 250 mg/5 mL suspension twice daily. 1
Treatment duration should be 10 days minimum for any infection, with continuation for 48-72 hours beyond symptom resolution. 11
High-Dose Regimen (Severe Infections or Resistance Risk Factors)
Escalate to high-dose amoxicillin (80-90 mg/kg/day divided twice daily) when any of the following risk factors are present: 234
- Age < 2 years
- Daycare attendance
- Antibiotic use within the past 30 days
- Moderate-to-severe illness presentation
- Community pneumococcal resistance > 10%
High-Dose Calculation for This Patient
For this 38.6 kg child, high-dose therapy equals approximately 1,737 mg twice daily (90 mg/kg/day), which is approximately 21.7 mL of the 400 mg/5 mL suspension per dose. 34
This high-dose regimen is specifically recommended for severe acute otitis media, community-acquired pneumonia, acute bacterial sinusitis with risk factors, and infections requiring enhanced pneumococcal coverage. 234
Infection-Specific Recommendations
Community-Acquired Pneumonia
- Prescribe 80-90 mg/kg/day divided twice daily (approximately 1,737 mg twice daily) for pneumococcal pneumonia or severe lower respiratory tract infections. 34
- Duration: 5-10 days depending on severity; shorter 5-day courses are adequate for uncomplicated outpatient pneumonia. 4
Acute Otitis Media
- Standard dose (45 mg/kg/day) is appropriate for children ≥ 2 years without recent antibiotic exposure or daycare attendance. 4
- High dose (80-90 mg/kg/day) is required if no clinical improvement after 48-72 hours or if risk factors are present. 34
Acute Bacterial Sinusitis
- For mild disease without risk factors: 45 mg/kg/day divided twice daily for 10 days. 24
- For moderate-to-severe illness or presence of risk factors: 80-90 mg/kg/day of amoxicillin-clavulanate (with 6.4 mg/kg/day clavulanate) divided twice daily. 23
Streptococcal Pharyngitis
- Prescribe 50 mg/kg/day divided twice daily (approximately 965 mg twice daily for this child) for a mandatory 10-day course to prevent acute rheumatic fever. 311
Critical Dosing Pitfalls to Avoid
Do not cap amoxicillin doses at "adult maximums" (e.g., 500 mg twice daily) for children > 15 kg, as this results in significant underdosing and treatment failure, especially against resistant organisms. 35
Research demonstrates that larger children systematically receive declining mg/kg doses, with most patients above 60 kg dosed at less than 40 mg/kg/day—well below guideline recommendations. 5
Weight-based dosing must be used to ensure therapeutic exposure; the FDA label does not specify an adult maximum dose cap for pediatric patients. 11
When to Escalate to Amoxicillin-Clavulanate
Switch to high-dose amoxicillin-clavulanate (80-90 mg/kg/day of amoxicillin component) when: 23
- No clinical improvement after 48-72 hours of amoxicillin monotherapy
- β-lactamase-producing Haemophilus influenzae or Moraxella catarrhalis is suspected
- Severe illness requiring broader antimicrobial coverage
- Local prevalence of β-lactamase-producing organisms is high