Amoxicillin Dosing for Strep Throat in a 40-Pound Child
For a 40-pound (18 kg) child with strep throat, prescribe amoxicillin 50 mg/kg/day divided into 2 doses for 10 days, which equals 450 mg twice daily (total 900 mg/day). 1
Weight-Based Calculation
- A 40-pound child weighs approximately 18 kg
- The American Academy of Pediatrics recommends 50-75 mg/kg/day divided into 2 doses for Group A Streptococcal pharyngitis 1
- At 50 mg/kg/day: 18 kg × 50 mg = 900 mg/day total, given as 450 mg twice daily 1
- At 75 mg/kg/day (upper range): 18 kg × 75 mg = 1,350 mg/day total, given as 675 mg twice daily 1
- The maximum daily dose should not exceed 1,000 mg per day for strep throat in children 1
Practical Prescribing
- Prescribe 500 mg twice daily (1,000 mg/day total), which falls within the 50-75 mg/kg/day range and respects the 1,000 mg/day maximum 1
- This can be given as amoxicillin suspension 250 mg/5 mL: give 10 mL (500 mg) twice daily 2
- Alternatively, use 400 mg/5 mL suspension: give 6.25 mL (500 mg) twice daily 2
Treatment Duration and Key Considerations
- Complete a full 10-day course to prevent acute rheumatic fever, even if symptoms resolve earlier 1, 2
- Amoxicillin should be taken at the start of a meal to minimize gastrointestinal intolerance 2
- If no improvement occurs within 48-72 hours, reevaluate the diagnosis 1
Common Pitfalls to Avoid
- Do not underdose: Using 40-45 mg/kg/day instead of the recommended 50-75 mg/kg/day is inadequate 3, 4
- Research demonstrates that amoxicillin 40 mg/kg/day achieved only 54.5% bacteriologic cure versus 79.3% with higher dosing 3
- Do not prescribe for less than 10 days for strep throat, as shorter courses increase relapse risk 1, 2