What is the appropriate amoxicillin dosage for a 13-year-old weighing 120 lb (approximately 54 kg)?

Amoxicillin Dosing for a 13-Year-Old Weighing 120 lbs

For a 13-year-old weighing 120 lbs (54 kg), the appropriate amoxicillin dosage depends on infection severity: for mild-to-moderate infections, prescribe 500 mg every 12 hours or 250 mg every 8 hours; for severe infections or lower respiratory tract infections, prescribe 875 mg every 12 hours or 500 mg every 8 hours. 1

Weight-Based Dosing Considerations

Since this patient weighs 54 kg (exceeding the 40 kg threshold), they should receive adult dosing regimens rather than pediatric weight-based calculations. 1

• The FDA-approved dosing for patients ≥40 kg follows standard adult dosing protocols, not mg/kg calculations 1
• This patient falls well above the 40 kg cutoff where pediatric weight-based dosing transitions to fixed adult doses 2, 1

Infection-Specific Dosing Recommendations

Mild-to-Moderate Infections

For ear/nose/throat, skin/skin structure, or genitourinary tract infections of mild-to-moderate severity:

• 500 mg every 12 hours OR 250 mg every 8 hours 1

Severe Infections

For severe infections or any lower respiratory tract infection:

• 875 mg every 12 hours OR 500 mg every 8 hours 1
• The higher dose ensures adequate coverage against potentially resistant organisms 2, 3

High-Dose Regimens for Resistant Pathogens

If drug-resistant Streptococcus pneumoniae or recent antibiotic exposure (within 4-6 weeks) is suspected:

• Consider 90 mg/kg/day divided every 12 hours, which would be approximately 2,400 mg/day (1,200 mg twice daily) for this 54 kg patient 4, 5
• This high-dose regimen is particularly important for acute otitis media or community-acquired pneumonia in regions with >10% penicillin-resistant pneumococci 4, 3

Treatment Duration

• Minimum 48-72 hours beyond symptom resolution or evidence of bacterial eradication 1
• 7-10 days for most infections, with duration based on clinical response 4, 6
• 10 days minimum for any Streptococcus pyogenes infection to prevent acute rheumatic fever 1
• Sinusitis may require up to 14 days depending on clinical response 4

Critical Pitfalls to Avoid

Do not underdose when resistant pathogens are suspected. Recent antibiotic exposure, daycare attendance, or treatment failure all warrant the 90 mg/kg/day high-dose regimen rather than standard dosing. 4

Ensure adequate treatment duration. Shorter courses (3 days) may result in slightly longer time to cough resolution compared to 7-day courses, though overall efficacy remains similar for uncomplicated infections. 6

Verify renal function. Although this patient's age makes severe renal impairment unlikely, patients with glomerular filtration rate <30 mL/min should NOT receive the 875 mg dose and require dosage adjustment. 1

Administration Instructions

• Administer at the start of a meal to minimize gastrointestinal intolerance 1
• For oral suspension (if used instead of tablets), shake well before each use and refrigerate after reconstitution (though not required) 1
• Any unused reconstituted suspension must be discarded after 14 days 1