What is the appropriate amoxicillin dosing for a 12-year-old (twelve years old) patient weighing 131 pounds?

Amoxicillin Dosing for a 12-Year-Old Weighing 131 lbs

For a 12-year-old weighing 131 lbs (59.5 kg), the appropriate amoxicillin dose depends on the indication, but for most common infections like Group A Streptococcus pharyngitis or mild community-acquired pneumonia, use 50-75 mg/kg/day divided into 2 doses, which translates to approximately 1,500-2,200 mg twice daily (maximum 3,000-4,500 mg/day total). 1

Weight Conversion and Calculation

• 131 lbs = 59.5 kg
• This patient exceeds typical pediatric weight bands and approaches adult dosing thresholds

Indication-Specific Dosing

For Group A Streptococcus (Pharyngitis/Pneumonia)

• Standard dose: 50-75 mg/kg/day divided into 2 doses 1
• For this patient: 2,975-4,462 mg/day total (approximately 1,500-2,200 mg twice daily)
• This is the preferred oral therapy for Group A Streptococcus infections 1

For Haemophilus influenzae (if β-lactamase negative)

• Dose: 75-100 mg/kg/day divided into 3 doses 1
• For this patient: 4,462-5,950 mg/day total (approximately 1,500-2,000 mg three times daily)
• If β-lactamase producing, switch to amoxicillin-clavulanate 1

For Community-Acquired Pneumonia (CAP)

• Recent high-quality evidence from the CAP-IT trial demonstrated that lower-dose amoxicillin (35-50 mg/kg/day) was non-inferior to higher-dose (70-90 mg/kg/day) for uncomplicated CAP 2
• For this patient: Lower dose = 2,082-2,975 mg/day; Higher dose = 4,165-5,355 mg/day
• However, the RETAPP trial showed amoxicillin at 50 mg/kg/dose twice daily (approximately 100 mg/kg/day total) was superior to placebo 3

Practical Dosing Recommendations

Most Common Infections (Pharyngitis, Otitis Media, Mild CAP)

• Prescribe: Amoxicillin 875 mg twice daily (total 1,750 mg/day = 29 mg/kg/day)
• This is a standard adult formulation that is appropriate for this weight 4
• Alternative: Amoxicillin 500 mg three times daily (total 1,500 mg/day = 25 mg/kg/day) 4

Moderate to Severe Infections or Resistant Organisms

• Prescribe: Amoxicillin 1,000 mg three times daily (total 3,000 mg/day = 50 mg/kg/day)
• This achieves the guideline-recommended 50-75 mg/kg/day range 1

High-Dose Therapy (for resistant Streptococcus pneumoniae or severe CAP)

• Prescribe: Amoxicillin 1,500-2,000 mg three times daily (total 4,500-6,000 mg/day = 75-100 mg/kg/day)
• This achieves optimal pharmacodynamic targets (40% time above MIC) 2, 5

Duration of Therapy

• For uncomplicated CAP: 3 days is non-inferior to 7 days for antibiotic retreatment rates, though 7 days resulted in slightly faster cough resolution 2
• For other infections: Follow standard duration recommendations (typically 7-10 days for pharyngitis, 5-7 days for sinusitis) 4

Important Caveats

Pharmacodynamic Considerations

• Amoxicillin efficacy depends on maintaining plasma concentrations above the MIC for ≥40% of the dosing interval 2, 5
• For organisms with MIC ≥4 mg/L, standard dosing may be insufficient, and higher doses or more frequent administration may be needed 5

Amoxicillin-Clavulanate Alternative

• If β-lactamase-producing organisms suspected (H. influenzae), use amoxicillin-clavulanate 45 mg/kg/day in 3 doses or 90 mg/kg/day in 2 doses (based on amoxicillin component) 1
• The 875/125 mg formulation twice daily is well-tolerated and as effective as 500/125 mg three times daily 4

Weight-Based vs. Adult Dosing

• At 59.5 kg, this patient is transitioning from pediatric to adult dosing
• Adult maximum doses (typically 3,000-4,000 mg/day) should not be exceeded without specific indication 6
• Weight-based calculations at the higher end may exceed typical adult doses, so cap at adult maximums unless treating resistant organisms