What is the appropriate amoxicillin dose for a 5‑year‑old child weighing 18.9 kg for acute bacterial infections?

Amoxicillin Dosing for a 5-Year-Old Weighing 18.9 kg

For a 5-year-old child weighing 18.9 kg, the appropriate amoxicillin dose depends on the specific infection being treated, but for most acute bacterial respiratory infections (including pneumonia, sinusitis, and otitis media), the recommended dose is 90 mg/kg/day divided into two doses every 12 hours, which equals approximately 850 mg twice daily (1,700 mg total daily dose). 1, 2

Standard High-Dose Regimen for Respiratory Infections

• For community-acquired pneumonia in children ≥5 years, the Infectious Diseases Society of America and Pediatric Infectious Diseases Society recommend 90 mg/kg/day divided into 2 doses (maximum 4 g/day) to ensure adequate coverage of penicillin-resistant Streptococcus pneumoniae. 1, 2

• For acute bacterial sinusitis in children ≥2 years without risk factors, 45 mg/kg/day divided every 12 hours is acceptable, but 80–90 mg/kg/day divided every 12 hours is required if the child is <2 years, attends daycare, received antibiotics in the past 30 days, or has moderate-to-severe illness. 1

• For acute otitis media, the American Academy of Pediatrics recommends 80–90 mg/kg/day to achieve middle ear fluid concentrations that overcome intermediately resistant S. pneumoniae (MIC 0.12–1.0 µg/mL) and many highly resistant serotypes (MIC ≥2 µg/mL). 1, 3

Practical Dosing Calculation for This Patient

• Weight-based calculation: 18.9 kg × 90 mg/kg/day = 1,701 mg total daily dose, administered as 850 mg every 12 hours. 1

• Using liquid suspension (400 mg/5 mL): Each dose of 850 mg = approximately 10.6 mL twice daily. 1

• Using liquid suspension (250 mg/5 mL): Each dose of 850 mg = approximately 17 mL twice daily. 1

Indication-Specific Dosing

Group A Streptococcal Pharyngitis

• Dose: 50–75 mg/kg/day divided into 2 doses for 10 days (not exceeding 1,000 mg per dose). 1
• For this 18.9 kg child: 945–1,418 mg daily, administered as 475–700 mg twice daily (capped at 500 mg per dose due to the 1,000 mg maximum). 1

Mild to Moderate Skin or Genitourinary Infections

• Dose: 25 mg/kg/day divided every 12 hours or 20 mg/kg/day divided every 8 hours. 4
• For this child: approximately 240 mg twice daily or 125 mg three times daily. 4

Treatment Duration and Monitoring

• Standard duration: 7–10 days for most respiratory infections; 10 days specifically for pneumonia and Group A streptococcal pharyngitis to prevent acute rheumatic fever. 1, 4

• Expected clinical improvement: Children on appropriate therapy should show reduced fever, improved respiratory effort, and better oral intake within 48–72 hours. 1, 2

• If no improvement after 48–72 hours: Reassess for complications (empyema, necrotizing pneumonia), consider atypical pathogens (Mycoplasma, Chlamydophila) and add azithromycin, or switch to amoxicillin-clavulanate if β-lactamase-producing organisms are suspected. 1, 2

When to Use Amoxicillin-Clavulanate Instead

• Switch to amoxicillin-clavulanate (90 mg/kg/day amoxicillin component + 6.4 mg/kg/day clavulanate, divided twice daily) if: 1, 3
  • The child received antibiotics within the past 30 days
  • The child is not fully immunized against Haemophilus influenzae type b
  • There is concurrent purulent conjunctivitis (otitis-conjunctivitis syndrome)
  • Treatment failure after 48–72 hours of amoxicillin monotherapy
  • Severe presentation at initial diagnosis

Common Pitfalls to Avoid

• Underdosing with 40–45 mg/kg/day is inadequate for resistant S. pneumoniae and is a common, dangerous error that leads to treatment failure. 1, 2, 5

• Using macrolides as first-line therapy for presumed bacterial pneumonia is inappropriate; amoxicillin remains the definitive first choice. 2

• Failing to consider MRSA in severe pneumonia with necrotizing infiltrates, empyema, or recent influenza requires adding vancomycin (40–60 mg/kg/day IV) or clindamycin (40 mg/kg/day IV). 2

• Stopping antibiotics early when symptoms improve before completing the full 10-day course increases the risk of relapse and rheumatic fever in streptococcal infections. 1, 4

Penicillin Allergy Alternatives

• For non-anaphylactic reactions: Cephalosporins (cefdinir, cefuroxime, cephalexin) have low cross-reactivity (1–3%) and are safe alternatives. 1, 2

• For severe (anaphylactic) reactions: Levofloxacin 16–20 mg/kg/day divided every 12 hours (maximum 750 mg/day) or azithromycin 10 mg/kg on day 1, then 5 mg/kg/day on days 2–5. 1, 2