What is the correct amoxicillin dosage for a 4‑month‑old infant weighing approximately 4 kg?

Amoxicillin Dosing for a 4-Month-Old Infant

For a 4-month-old infant weighing approximately 4 kg, the recommended amoxicillin dose is 30 mg/kg/day divided every 12 hours (60 mg per dose, twice daily), which translates to approximately 1.2 mL of 125 mg/5 mL suspension per dose. 1

Age-Specific Dosing Considerations

Infants Under 3 Months (12 Weeks)

• The FDA label explicitly states that for pediatric patients less than 12 weeks (3 months) of age, the recommended upper dose is 30 mg/kg/day divided every 12 hours due to incompletely developed renal function affecting amoxicillin elimination. 1

• A 4-month-old infant falls just outside this restriction, but given the proximity to the 3-month cutoff and the low weight (4 kg suggests possible prematurity or growth concerns), the conservative 30 mg/kg/day dosing remains appropriate unless treating a severe infection. 1

• Research in neonates and premature infants demonstrated that 50 mg/kg/day administered every 12 hours achieved adequate serum levels and was well tolerated, but this was studied primarily in the first days of life. 2

Standard Dosing for Infants ≥3 Months

• For infants 3 months and older weighing less than 40 kg, standard dosing for mild-to-moderate infections is 25 mg/kg/day divided every 12 hours or 20 mg/kg/day divided every 8 hours. 1

• For severe infections or lower respiratory tract infections in this age group, the dose increases to 45 mg/kg/day divided every 12 hours or 40 mg/kg/day divided every 8 hours. 1

Practical Dosing Calculation for This Patient

For Mild-to-Moderate Infections

• Using 25 mg/kg/day: 4 kg × 25 mg/kg = 100 mg total daily dose
• Divided every 12 hours = 50 mg per dose (approximately 1 mL of 125 mg/5 mL suspension twice daily)

For Severe Infections or Respiratory Tract Involvement

• Using 45 mg/kg/day: 4 kg × 45 mg/kg = 180 mg total daily dose
• Divided every 12 hours = 90 mg per dose (approximately 1.8 mL of 125 mg/5 mL suspension twice daily)

Conservative Approach Given Age and Weight

• Given this infant's very low weight and young age, starting with 30 mg/kg/day (120 mg total daily, 60 mg per dose = 1.2 mL twice daily) represents a middle ground between neonatal dosing and standard pediatric dosing. 1

Indication-Specific Guidance

Community-Acquired Pneumonia

• For infants under 5 years with presumed bacterial pneumonia, guidelines recommend 90 mg/kg/day of amoxicillin divided into 2 doses, but this applies to older, heavier infants without renal maturation concerns. 3
• For a 4-month-old weighing only 4 kg, this would require specialist consultation before prescribing such high doses.

Acute Otitis Media

• High-dose amoxicillin (80-90 mg/kg/day) is recommended for children under 2 years with acute otitis media, but again, the very low weight and young age warrant caution. 3, 4
• If treating otitis media in this patient, consider starting at 45 mg/kg/day (180 mg/day total) rather than the full 90 mg/kg/day dose.

Group A Streptococcal Infections

• For streptococcal pharyngitis or skin infections, the recommended dose is 50-75 mg/kg/day divided into 2 doses for 10 days, which would be 200-300 mg/day total for this 4 kg infant. 3

Treatment Duration and Monitoring

• Continue treatment for a minimum of 48-72 hours beyond symptom resolution or evidence of bacterial eradication. 1

• For any infection caused by Streptococcus pyogenes, treat for at least 10 days to prevent acute rheumatic fever. 1

• Clinical improvement should be evident within 48-72 hours; if no improvement occurs, reevaluation and possible change in therapy are necessary. 3

Critical Safety Considerations

Renal Function Monitoring

• Incompletely developed renal function in infants under 3 months necessitates the lower 30 mg/kg/day maximum dose; a 4-month-old is transitioning out of this high-risk period but may still have immature clearance, especially if premature or small-for-gestational-age. 1

When to Use Amoxicillin-Clavulanate Instead

• Switch to amoxicillin-clavulanate if β-lactamase-producing organisms (H. influenzae, M. catarrhalis) are suspected, particularly if the infant has incomplete Haemophilus influenzae type b vaccination. 3, 4

• For infants under 1 year, amoxicillin-clavulanate dosing is 2.5 mL of 125/31 suspension three times daily per standard age-based guidelines, though weight-based calculation is preferred. 5

Reconstitution and Storage

• After reconstitution, shake the oral suspension well before each use and keep the bottle tightly closed. 1

• Discard any unused suspension after 14 days; refrigeration is preferable but not required. 1

• The required dose should be placed directly on the infant's tongue or mixed with formula, milk, or water and administered immediately. 1

Common Pitfalls to Avoid

• Do not use adult dosing formulations or calculations for infants; always use weight-based pediatric dosing with age-appropriate maximum limits. 1

• Do not exceed 30 mg/kg/day in infants very close to 3 months of age without clear indication for higher dosing and consideration of renal maturation. 1

• Do not prescribe amoxicillin for viral infections; ensure bacterial infection is proven or strongly suspected. 1

• Do not discontinue antibiotics early even if symptoms improve; complete the full prescribed course. 3

• For a 4 kg infant at 4 months of age, verify that weight is appropriate for age; significant growth faltering may indicate underlying conditions affecting drug metabolism and clearance. 6