What is the appropriate amoxicillin dose for a 13‑year‑old patient weighing 90 kg?

Amoxicillin Dosing for a 90 kg, 13-Year-Old Patient

For a 13-year-old weighing 90 kg, prescribe the standard adult dose of amoxicillin rather than weight-based pediatric dosing, because children weighing ≥40 kg should be dosed as adults. 1

Weight-Based Dosing Threshold

• Children weighing more than 40 kg should be dosed using standard adult formulations and dosing regimens, not pediatric weight-based calculations. 1
• At 90 kg, this patient is well above the 40 kg threshold that marks the transition from pediatric to adult dosing. 1

Standard Adult Dosing Regimens by Indication

For Mild to Moderate Respiratory Tract Infections

• 500 mg every 12 hours or 250 mg every 8 hours. 2

For Severe Respiratory Tract Infections or Community-Acquired Pneumonia

• 875 mg every 12 hours or 500 mg every 8 hours. 2

For Group A Streptococcal Pharyngitis

• 500 mg twice daily for 10 days (maximum single dose 1,000 mg). 1
• Alternatively, 1,000 mg once daily for 10 days may improve adherence. 1

For Skin/Skin Structure or Genitourinary Tract Infections

• Mild/moderate: 500 mg every 12 hours or 250 mg every 8 hours. 2
• Severe: 875 mg every 12 hours or 500 mg every 8 hours. 2

Maximum Daily Dose Considerations

• The maximum daily dose of amoxicillin is 4,000 mg/day regardless of weight. 3, 1
• Even though weight-based pediatric dosing (90 mg/kg/day) would theoretically yield 8,100 mg/day for a 90 kg patient, this calculation is not applicable because the patient exceeds 40 kg and should receive adult dosing. 1
• The FDA-approved maximum adult dose is effectively capped at 1,750 mg/day (875 mg twice daily) for standard infections or 3,000 mg/day (1,000 mg three times daily) for H. pylori eradication. 2

Treatment Duration

• Continue treatment for a minimum of 48–72 hours beyond symptom resolution or evidence of bacterial eradication. 2
• For Group A streptococcal infections, treat for at least 10 days to prevent acute rheumatic fever. 2
• For community-acquired pneumonia, treat for 10 days total. 1

Clinical Monitoring

• Clinical improvement should be evident within 48–72 hours of initiating therapy. 1, 4
• If no improvement or worsening occurs after 48–72 hours, reassess the diagnosis and consider treatment failure, resistant organisms, or complications. 4

Common Pitfall to Avoid

• Do not apply pediatric weight-based dosing (e.g., 90 mg/kg/day) to adolescents ≥40 kg, as this leads to inappropriately high doses that exceed FDA-approved adult maximums and may increase adverse effects without additional benefit. 1, 5
• The 40 kg threshold is explicitly established in tuberculosis and other treatment guidelines as the transition point to adult dosing. 1

When to Consider Amoxicillin-Clavulanate Instead

• Switch to amoxicillin-clavulanate 875/125 mg twice daily if the patient has:
  • Recent antibiotic exposure within the past 30 days. 1
  • Treatment failure after 48–72 hours on amoxicillin alone. 4
  • Suspected β-lactamase-producing organisms (e.g., H. influenzae, M. catarrhalis). 1