Amoxicillin Dosing for a 3-Year-Old Child Weighing 18 kg
For a 3-year-old child weighing 18 kg, the appropriate amoxicillin dose depends on the infection severity and type: for mild to moderate ear/nose/throat, skin, or genitourinary infections, prescribe 450 mg/day divided into 225 mg every 12 hours (25 mg/kg/day) or 300 mg/day divided into 100 mg every 8 hours (20 mg/kg/day); for severe infections or any lower respiratory tract infection, prescribe 810 mg/day divided into 405 mg every 12 hours (45 mg/kg/day) or 720 mg/day divided into 240 mg every 8 hours (40 mg/kg/day). 1
Dosing Algorithm by Infection Type
For Mild to Moderate Infections
(Ear/nose/throat, skin/skin structure, genitourinary tract)
- Standard dose: 25 mg/kg/day divided every 12 hours = 225 mg twice daily
- Alternative: 20 mg/kg/day divided every 8 hours = 120 mg three times daily
For this 18 kg child:
- 18 kg × 25 mg/kg/day = 450 mg/day → 225 mg every 12 hours
- OR 18 kg × 20 mg/kg/day = 360 mg/day → 120 mg every 8 hours
For Severe Infections or Lower Respiratory Tract Infections
(Including pneumonia, regardless of severity)
- High dose: 45 mg/kg/day divided every 12 hours = 405 mg twice daily
- Alternative: 40 mg/kg/day divided every 8 hours = 240 mg three times daily
For this 18 kg child:
- 18 kg × 45 mg/kg/day = 810 mg/day → 405 mg every 12 hours
- OR 18 kg × 40 mg/kg/day = 720 mg/day → 240 mg every 8 hours
Context-Specific Recommendations
For Community-Acquired Pneumonia (CAP)
Use high-dose amoxicillin (90 mg/kg/day in 2 doses or 45 mg/kg/day in 3 doses) for suspected or confirmed Streptococcus pneumoniae with penicillin MICs <2.0 µg/mL. 2 This translates to:
- 18 kg × 90 mg/kg/day = 1,620 mg/day → 810 mg every 12 hours
- OR 18 kg × 45 mg/kg/day = 810 mg/day → 270 mg every 8 hours
The higher 90 mg/kg/day dosing (twice daily) is preferred for pneumococcal pneumonia given increasing intermediate penicillin resistance patterns. 3
For Group A Streptococcal Infections
Use 50-75 mg/kg/day divided into 2 doses. 2 For this 18 kg child:
- 18 kg × 50-75 mg/kg/day = 900-1,350 mg/day → 450-675 mg every 12 hours
For Haemophilus influenzae (β-lactamase negative)
Use 75-100 mg/kg/day divided into 3 doses. 4 For this 18 kg child:
- 18 kg × 75-100 mg/kg/day = 1,350-1,800 mg/day → 450-600 mg every 8 hours
Critical Dosing Considerations
Duration of Therapy
- Continue treatment for minimum 48-72 hours beyond symptom resolution or bacterial eradication 1
- For Streptococcus pyogenes infections: minimum 10 days to prevent acute rheumatic fever 1
Administration
- Give at the start of a meal to minimize gastrointestinal intolerance 1
- Oral suspension can be mixed with formula, milk, fruit juice, water, or cold drinks; consume immediately after mixing 1
Common Pitfall: Weight-Based Dosing Disparity
Research demonstrates that larger children systematically receive lower mg/kg doses than recommended, with most children >60 kg receiving <40 mg/kg/day. 5 At 18 kg, this child is in the weight range where appropriate dosing is typically maintained, but prescribers should verify the calculated total daily dose matches guideline recommendations rather than defaulting to lower doses.
Practical Formulation Selection
For a child requiring 810 mg/day (high-dose regimen):
- Using 250 mg/5 mL suspension: Give 8 mL (400 mg) every 12 hours
- Using 400 mg chewable tablets: Give 1 tablet (400 mg) every 12 hours
For a child requiring 450 mg/day (standard-dose regimen):
- Using 250 mg/5 mL suspension: Give 4.5 mL (225 mg) every 12 hours
- Using 125 mg/5 mL suspension: Give 9 mL (225 mg) every 12 hours
Storage After Reconstitution
- Reconstituted suspension must be discarded after 14 days 1
- Refrigeration is preferable but not required
- Shake well before each use