Amoxicillin Dosing for 9-Month-Old, 8 kg Infant
For a 9-month-old infant weighing 8 kg, administer amoxicillin 40 mg/kg/day divided into two doses (160 mg every 12 hours, or 1.6 mL of 100 mg/mL suspension twice daily).
Dosing Rationale
The FDA-approved dosing for infants ≥3 months with lower respiratory tract infections is 45 mg/kg/day divided every 12 hours or 40 mg/kg/day divided every 8 hours 1. For this 8 kg infant:
- Lower respiratory tract infections (mild/moderate or severe): 45 mg/kg/day = 360 mg/day (180 mg every 12 hours = 1.8 mL twice daily) OR 40 mg/kg/day = 320 mg/day (approximately 160 mg every 8 hours = 1.6 mL three times daily)
- Ear/nose/throat or skin infections (severe): Same dosing applies
The IDSA/PIDS guidelines 2 recommend amoxicillin 90 mg/kg/day in 2 doses or 45 mg/kg/day in 3 doses for oral therapy of pneumococcal pneumonia in children >3 months. However, these higher doses target resistant Streptococcus pneumoniae with penicillin MICs >2.0 μg/mL.
Practical Dosing Approach
For empiric treatment without known resistance:
- Start with 40-45 mg/kg/day divided twice daily (320-360 mg/day = 1.6-1.8 mL twice daily)
- This provides 160-180 mg per dose every 12 hours
If treating confirmed or suspected resistant pneumococcus:
- Use 90 mg/kg/day divided twice daily (720 mg/day = 3.6 mL twice daily)
- This provides 360 mg per dose every 12 hours
Key Clinical Considerations
Twice-daily dosing is preferred over three-times-daily for several reasons:
- Research demonstrates equivalent efficacy between 50 mg/kg/day given twice versus three times daily 3
- Improved compliance with less frequent dosing 4
- Pharmacokinetic studies show adequate drug levels maintained with twice-daily regimens 4
Important caveats:
- For infants <3 months (12 weeks), maximum dose is 30 mg/kg/day divided every 12 hours due to immature renal function 1
- This 9-month-old infant falls well above this age threshold
- Always administer at the start of meals to minimize gastrointestinal intolerance 1
- Shake suspension well before each dose
- Discard unused suspension after 14 days; refrigeration preferred but not required 1
Duration of therapy:
- Continue for minimum 48-72 hours beyond symptom resolution 1
- For Streptococcus pyogenes infections, treat for at least 10 days to prevent rheumatic fever 1
The lower FDA-approved doses (40-45 mg/kg/day) remain appropriate for most community-acquired infections in areas without high-level penicillin resistance 5. The higher IDSA/PIDS doses (90 mg/kg/day) should be reserved for documented or strongly suspected resistant pneumococcal disease 2.