Amoxicillin Dosing for an 11-Year-Old Weighing 113 lb (51.3 kg)
For this 11-year-old child weighing 113 lb (51.3 kg), prescribe amoxicillin 500 mg (6.25 mL of the 400 mg/5 mL suspension) every 12 hours for most common infections, or 875 mg (approximately 11 mL) every 12 hours for severe infections or pneumonia.
Weight-Based Dosing Calculation
This child weighs 51.3 kg, which exceeds the 40 kg threshold where FDA labeling transitions from weight-based to fixed adult dosing 1.
Standard Dosing Options:
For mild to moderate infections (otitis media, sinusitis, skin infections):
- Target dose: 25 mg/kg/day divided every 12 hours
- Calculation: 51.3 kg × 25 mg/kg/day = 1,283 mg/day
- Practical dose: 500 mg every 12 hours (1,000 mg/day)
- Using 400 mg/5 mL suspension: 6.25 mL every 12 hours
For severe infections or pneumonia:
- Target dose: 45 mg/kg/day divided every 12 hours
- Calculation: 51.3 kg × 45 mg/kg/day = 2,309 mg/day
- Practical dose: 875 mg every 12 hours (1,750 mg/day) 1
- Using 400 mg/5 mL suspension: approximately 11 mL every 12 hours
Clinical Context Matters
The FDA label specifies that for children ≥40 kg, adult dosing applies: 500 mg every 12 hours for mild/moderate infections or 875 mg every 12 hours for severe infections 1.
Important caveat: The 400 mg/5 mL suspension formulation makes delivering 875 mg doses cumbersome (requires 10.9 mL). Consider switching to:
- Tablets: 875 mg tablets are easier for an 11-year-old
- Alternative suspension: 250 mg/5 mL would require 17.5 mL per dose (still awkward)
The Dosing Disparity Problem
Research demonstrates that larger children systematically receive lower mg/kg doses than recommended. One study found children >60 kg typically receive <40 mg/kg/day, while children ≤6 years cluster around 80 mg/kg/day 2. This child, at 51.3 kg, falls into the vulnerable zone where clinicians often underdose due to reluctance to exceed "adult maximum doses" 3.
Do not fall into this trap. The 500 mg or 875 mg doses are appropriate and necessary for adequate tissue penetration and bacterial eradication 4, 5.
Duration of Therapy
- Standard duration: 5-7 days for most respiratory infections 6, 7
- Pneumonia: Recent evidence shows 3 days may be non-inferior to 7 days for uncomplicated cases, though cough resolution is slightly faster with 7 days 8
- Streptococcal pharyngitis: Minimum 10 days to prevent rheumatic fever 1
Practical Administration
- Give at the start of meals to minimize GI intolerance 1
- Shake suspension well before each dose
- Refrigeration preferred but not required
- Discard unused suspension after 14 days 1
Red Flags Requiring Dose Adjustment
Do NOT use 875 mg dose if:
- Renal impairment with GFR <30 mL/min (use 500 mg every 12 hours or 250 mg every 12 hours depending on severity) 1
Consider higher dosing (80-90 mg/kg/day) if:
- Documented or suspected resistant Streptococcus pneumoniae (MIC ≥2 mg/L) 9, 10, 5
- Recent antibiotic exposure
- Daycare attendance
- Geographic area with high resistance rates
For this scenario with resistant pneumococcus, the dose would be approximately 4,100 mg/day (51.3 kg × 80 mg/kg), but this exceeds practical limits. In such cases, switch to amoxicillin-clavulanate or consider alternative antibiotics per IDSA guidelines 9, 10.