What dose of amoxicillin (400 mg/5 ml suspension) should be given to a 40‑lb (≈18 kg) child?

Amoxicillin Dosing for a 40-Pound Child

For a 40-pound (18 kg) child, the appropriate dose of amoxicillin 400 mg/5 mL suspension depends on the infection severity and type: for mild-to-moderate ear/nose/throat, skin, or genitourinary infections, give 450 mg (5.6 mL) every 12 hours; for severe infections or any lower respiratory tract infection, give 810 mg (10.1 mL) every 12 hours. 1

Standard Dosing by Infection Severity

The FDA-approved dosing for pediatric patients weighing less than 40 kg is weight-based and varies by infection type and severity 1:

Mild-to-Moderate Infections

• Ear/nose/throat, skin/skin structure, or genitourinary tract infections:
  • 25 mg/kg/day divided every 12 hours 1
  • For an 18 kg child: 450 mg per dose (5.6 mL of 400 mg/5 mL suspension) every 12 hours
  • Alternative: 20 mg/kg/day divided every 8 hours = 360 mg (4.5 mL) every 8 hours 1

Severe Infections or Lower Respiratory Tract Infections

• All lower respiratory tract infections (regardless of severity) and severe ear/nose/throat, skin, or genitourinary infections:
  • 45 mg/kg/day divided every 12 hours 1
  • For an 18 kg child: 810 mg per dose (10.1 mL of 400 mg/5 mL suspension) every 12 hours
  • Alternative: 40 mg/kg/day divided every 8 hours = 720 mg (9 mL) every 8 hours 1

High-Dose Amoxicillin for Resistant Organisms

In communities with high rates of penicillin-nonsusceptible Streptococcus pneumoniae (>10%), or for children with risk factors for resistance (age <2 years, daycare attendance, or antibiotic use within 30 days), higher doses are recommended 2:

• 80-90 mg/kg/day divided every 12 hours 2
• For an 18 kg child: 1440-1620 mg per day = 720-810 mg (9-10.1 mL) every 12 hours
• Maximum single dose: 2000 mg 2

This high-dose regimen is specifically recommended for acute bacterial sinusitis and community-acquired pneumonia when resistant organisms are suspected 2.

Important Clinical Considerations

Administration

• Give at the start of a meal to minimize gastrointestinal intolerance 1
• Shake suspension well before each use 1
• Suspension can be placed directly on the child's tongue or mixed with formula, milk, juice, or water (must be taken immediately) 1

Duration of Therapy

• Continue for 48-72 hours beyond symptom resolution or evidence of bacterial eradication 1
• For Streptococcus pyogenes (Group A Strep) infections: minimum 10 days to prevent acute rheumatic fever 1
• Recent evidence suggests 3-day courses may be noninferior to 7-day courses for uncomplicated community-acquired pneumonia, though cough resolution may be slightly delayed 3, 4

Common Pitfalls

• Underdosing larger children: Studies show clinicians often prescribe declining mg/kg doses as children get older/heavier, potentially leading to treatment failure in adolescents 5
• Viral coinfection: Amoxicillin middle ear fluid penetration is reduced during viral coinfection, supporting the use of higher doses when viral illness is present 6
• Storage: Reconstituted suspension expires after 14 days; refrigeration is preferable but not required 1

When to Use Amoxicillin-Clavulanate Instead

Consider switching to amoxicillin-clavulanate (high-dose: 90 mg/kg/day of amoxicillin component) for 2:

• Children <2 years old
• Recent antibiotic exposure (within 30 days)
• Daycare attendance
• Moderate-to-severe illness
• Areas with high β-lactamase-producing H. influenzae prevalence