Amoxicillin Dosing for a 40-Pound Child
For a 40-pound (18 kg) child, administer 360 mg of amoxicillin twice daily (every 12 hours) for mild to moderate infections, which equals 4.5 mL of the 400 mg/5 mL suspension per dose. 1
Weight-Based Dosing Calculation
- A 40-pound child weighs approximately 18 kg (1 kg = 2.2 lbs) 1
- The standard dose for most common bacterial infections is 45 mg/kg/day divided into two doses (every 12 hours) 1, 2
- This calculates to: 18 kg × 45 mg/kg/day = 810 mg/day total, or 405 mg per dose twice daily 1
- Using the 400 mg/5 mL suspension: 405 mg ÷ 400 mg × 5 mL = approximately 5 mL per dose 2
Dosing by Infection Severity
For mild to moderate infections (ear/nose/throat, skin, genitourinary):
- 25 mg/kg/day divided every 12 hours (or 20 mg/kg/day divided every 8 hours) 2
- For this 18 kg child: 450 mg/day total = 225 mg per dose twice daily = 2.8 mL per dose 2
For severe infections or lower respiratory tract infections:
- 45 mg/kg/day divided every 12 hours (or 40 mg/kg/day divided every 8 hours) 2
- For this 18 kg child: 810 mg/day total = 405 mg per dose twice daily = 5 mL per dose 2
For high-dose therapy (resistant organisms, community-acquired pneumonia):
- 90 mg/kg/day divided every 12 hours 1, 3
- For this 18 kg child: 1,620 mg/day total = 810 mg per dose twice daily = 10 mL per dose 1
Clinical Context and Recommendations
- The American Academy of Pediatrics recommends the higher dose (45 mg/kg/day) for respiratory infections to ensure adequate coverage against potentially resistant Streptococcus pneumoniae 3
- For otitis media specifically, the recommended dose is 40-45 mg/kg/day in 2 divided doses 1
- For streptococcal pharyngitis, use 50-75 mg/kg/day in 2 doses 1
- Avoid underdosing, which can lead to treatment failure and promote antimicrobial resistance 1
Administration Instructions
- Administer at the start of a meal to minimize gastrointestinal intolerance 2
- Shake the oral suspension well before each use 2
- Doses should be given approximately 12 hours apart for twice-daily regimens 4
- The reconstituted suspension must be discarded after 14 days; refrigeration is preferable but not required 2
Treatment Duration and Monitoring
- Most infections require 5-7 days of treatment, with a minimum of 48-72 hours beyond symptom resolution 1, 2
- For Streptococcus pyogenes infections, at least 10 days of treatment is required to prevent acute rheumatic fever 2
- Clinical improvement should occur within 48-72 hours of starting therapy 1, 3
- If no improvement is seen within 48-72 hours, consider treatment failure, resistant organisms, or alternative diagnoses 1, 3
Important Caveats
- Children weighing less than 40 kg should not receive the 875 mg tablet formulation 2
- For children with severe renal impairment (GFR <30 mL/min), dosing adjustments are required 2
- Complete the full course of antibiotics even if symptoms resolve earlier to prevent resistance and recurrence 1
- If treatment failure occurs after 48-72 hours, consider switching to amoxicillin-clavulanate at 90 mg/kg/day (amoxicillin component) 1