What is the appropriate amoxicillin dosing regimen (dose, frequency, formulation, and duration) for an infant (e.g., 10 kg) with mild infections versus severe infections such as pneumonia or otitis media, assuming normal renal function and no penicillin allergy?

Amoxicillin Dosing for Infants

For a 10 kg infant with mild infections (e.g., uncomplicated upper respiratory tract infections, skin infections), prescribe amoxicillin 25 mg/kg/day divided every 12 hours (125 mg twice daily), whereas for severe infections such as pneumonia or complicated otitis media, prescribe 45 mg/kg/day divided every 12 hours (225 mg twice daily). 1, 2

Standard Dosing Algorithm by Infection Severity

Mild to Moderate Infections

• For ear, nose, throat, skin, or genitourinary infections in infants ≥3 months and <40 kg, the FDA-approved dose is 25 mg/kg/day divided every 12 hours (or 20 mg/kg/day divided every 8 hours). 2
• For a 10 kg infant, this translates to 125 mg twice daily or approximately 3.1 mL of 200 mg/5 mL suspension twice daily. 1
• This regimen provides adequate coverage for susceptible Streptococcus pneumoniae, non-β-lactamase-producing Haemophilus influenzae, and Streptococcus pyogenes. 1

Severe Infections or Pneumonia

• For lower respiratory tract infections (pneumonia) or severe infections, the American Academy of Pediatrics and IDSA recommend 45 mg/kg/day divided every 12 hours (or 40 mg/kg/day divided every 8 hours). 1, 3, 2
• For a 10 kg infant, this translates to 225 mg twice daily or approximately 5.6 mL of 200 mg/5 mL suspension twice daily. 1
• This higher dose is essential to overcome penicillin-resistant Streptococcus pneumoniae with MICs up to 2 mg/L. 1, 3

High-Dose Regimen for Resistant Organisms

• When pneumococcal resistance is high (>10% penicillin-resistant S. pneumoniae), recent antibiotic exposure within 30 days, daycare attendance, age <2 years, or severe presentation, escalate to 90 mg/kg/day divided every 12 hours (maximum 4 g/day). 1, 3
• For a 10 kg infant, this translates to 450 mg twice daily or approximately 11.3 mL of 200 mg/5 mL suspension twice daily. 1
• This high-dose regimen achieves middle ear and sinus fluid concentrations sufficient to eradicate resistant pneumococci. 1

Indication-Specific Dosing

Community-Acquired Pneumonia

• Children <5 years with presumed bacterial pneumonia: amoxicillin 90 mg/kg/day in 2 doses (450 mg twice daily for 10 kg infant) for 5–7 days. 1, 3, 4
• Fully immunized, low-risk outpatients: may use 45 mg/kg/day in 2 doses (225 mg twice daily for 10 kg infant). 3
• Not fully immunized or concurrent purulent otitis media: use amoxicillin-clavulanate 90 mg/kg/day of amoxicillin component in 2 doses instead of plain amoxicillin to cover β-lactamase-producing H. influenzae. 1, 3

Acute Otitis Media

• Standard dose for children ≥2 years without risk factors: 45 mg/kg/day in 2 doses (225 mg twice daily for 10 kg infant). 1
• High-dose for children <2 years, daycare attendees, or recent antibiotic use: 80–90 mg/kg/day in 2 doses (400–450 mg twice daily for 10 kg infant). 1
• Research shows that standard-dose amoxicillin (40–45 mg/kg/day) achieves middle ear fluid concentrations <2 µg/mL for >50% of the dosing interval in many children, which is inadequate for resistant pneumococci. 5

Group A Streptococcal Pharyngitis

• Recommended dose: 50–75 mg/kg/day in 2 doses for 10 days (250–375 mg twice daily for 10 kg infant, not exceeding 1000 mg per dose). 1
• Alternative once-daily regimen: 50 mg/kg once daily (500 mg once daily for 10 kg infant, maximum 1000 mg) for 10 days to improve adherence. 1
• Complete the full 10-day course regardless of symptom improvement to prevent acute rheumatic fever. 1, 2

Special Populations

Infants <3 Months (12 Weeks)

• Maximum dose is 30 mg/kg/day divided every 12 hours (150 mg twice daily for a 10 kg infant, though most infants <3 months weigh <10 kg). 1, 2
• This lower dose accounts for incompletely developed renal function affecting amoxicillin elimination. 2
• For infants <3 months with pneumonia or serious bacterial infection, hospitalization for parenteral therapy is strongly preferred. 3

β-Lactamase-Producing Organisms

• When H. influenzae or Moraxella catarrhalis (β-lactamase producers) are suspected, switch to amoxicillin-clavulanate 90 mg/kg/day of amoxicillin component in 2 doses. 1, 3
• For a 10 kg infant, this is 450 mg of amoxicillin component twice daily. 1
• The 14:1 ratio formulation (amoxicillin:clavulanate) minimizes diarrhea while providing adequate β-lactamase inhibition. 1

Treatment Duration and Monitoring

• Continue treatment for a minimum of 48–72 hours beyond symptom resolution or evidence of bacterial eradication. 2
• For Streptococcus pyogenes infections, treat for at least 10 days to prevent acute rheumatic fever. 1, 2
• For pneumonia, treat for 10 days total, with reassessment if no improvement within 48–72 hours. 1, 3
• Children on appropriate therapy should demonstrate clinical improvement within 48–72 hours; failure to improve warrants reevaluation for complications (empyema, necrotizing pneumonia), resistant organisms (MRSA), or alternative diagnoses. 1, 3

Common Pitfalls to Avoid

• Underdosing with 40–45 mg/kg/day for pneumonia is a dangerous error that fails to overcome pneumococcal resistance; always use 90 mg/kg/day for presumed bacterial pneumonia. 1, 3
• Using plain amoxicillin in unimmunized children or those with concurrent purulent otitis media misses β-lactamase-producing H. influenzae; use amoxicillin-clavulanate instead. 1, 3
• Prescribing macrolides as first-line therapy for presumed bacterial pneumonia is inappropriate; amoxicillin is the definitive first-line agent. 3
• Failing to consider MRSA in severe pneumonia with necrotizing infiltrates, empyema, or recent influenza requires adding vancomycin or clindamycin to β-lactam therapy. 3

Practical Administration

• Administer amoxicillin at the start of a meal to minimize gastrointestinal intolerance. 2
• Shake oral suspension well before each use; refrigeration is preferable but not required. 2
• Discard any unused reconstituted suspension after 14 days. 2
• The suspension may be mixed with formula, milk, fruit juice, or water and should be taken immediately. 2