What is the correct dosing calculation for liquid amoxicillin (amoxicillin) in pediatric patients?

Calculating Liquid Amoxicillin Dosing for Pediatric Patients

For pediatric patients, calculate liquid amoxicillin dosing based on weight (mg/kg/day) rather than age, using 45 mg/kg/day divided every 12 hours for mild-to-moderate infections and 90 mg/kg/day divided every 12 hours for severe infections or high pneumococcal resistance areas, with a maximum daily dose of 4000 mg. 1, 2

Step-by-Step Dosing Calculation Algorithm

Step 1: Obtain Accurate Weight

• Always use the child's current weight in kilograms to calculate the dose, as weight-based dosing is more accurate than age-based dosing 2, 3

Step 2: Determine Infection Severity and Select Daily Dose

For mild-to-moderate infections (most common respiratory infections, skin infections, uncomplicated UTIs):

• Use 45 mg/kg/day divided every 12 hours 1, 2, 4
• Alternative: 40 mg/kg/day divided every 8 hours 4

For severe infections or high pneumococcal resistance areas:

• Use 90 mg/kg/day divided every 12 hours 1, 2
• Alternative: 80 mg/kg/day divided every 8 hours 4

For Group A Streptococcal infections (including scarlet fever):

• Use 50-75 mg/kg/day divided every 12 hours for 10 days 1, 2
• Maximum 1000 mg per dose 2

Step 3: Calculate Individual Dose

Formula:

• Twice-daily dosing: (Total daily dose in mg) ÷ 2 = dose per administration
• Three-times-daily dosing: (Total daily dose in mg) ÷ 3 = dose per administration

Step 4: Convert to Liquid Volume

Standard suspension concentrations:

• 125 mg/5 mL (25 mg/mL) 4
• 250 mg/5 mL (50 mg/mL) 4

Conversion formula:

• Volume (mL) = Dose (mg) ÷ Concentration (mg/mL)

Step 5: Apply Maximum Dose Limits

• Maximum daily dose: 4000 mg (4 g) per day 1, 2
• For children ≥40 kg with mild-moderate infections: use adult dosing of 500 mg every 12 hours or 250 mg every 8 hours 2, 4
• For children ≥40 kg with severe infections: use 875 mg every 12 hours or 500 mg every 8 hours 2, 4

Practical Dosing Examples

Example 1: 10 kg child with mild pneumonia

• Calculation: 10 kg × 45 mg/kg/day = 450 mg/day
• Per dose (BID): 450 mg ÷ 2 = 225 mg per dose
• Using 250 mg/5 mL suspension: 225 mg ÷ 50 mg/mL = 4.5 mL twice daily 1

Example 2: 5.5 kg infant (3 months) with severe pneumonia

• Calculation: 5.5 kg × 90 mg/kg/day = 495 mg/day
• Per dose (BID): 495 mg ÷ 2 = 247.5 mg per dose
• Using 250 mg/5 mL suspension: 247.5 mg ÷ 50 mg/mL = 2.5 mL twice daily 1

Example 3: 13.6 kg child with respiratory infection

• Calculation: 13.6 kg × 45 mg/kg/day = 612 mg/day
• Per dose (BID): 612 mg ÷ 2 = 306 mg per dose
• Using 250 mg/5 mL suspension: 306 mg ÷ 50 mg/mL = 6.1 mL twice daily 1

Special Population Considerations

Infants <3 Months (12 Weeks)

• Maximum dose: 30 mg/kg/day divided every 12 hours due to immature renal function 4
• This is a critical safety consideration that must not be exceeded 4

Renal Impairment (Children >3 months and >40 kg)

• GFR 10-30 mL/min: 500 mg or 250 mg every 12 hours 2, 4
• GFR <10 mL/min: 500 mg or 250 mg every 24 hours 2, 4
• Do NOT use 875 mg dose in severe renal impairment 4

Critical Prescription Requirements

Every amoxicillin prescription must include: 2

• Total daily dose in mg/kg/day
• Number of divided doses per day (e.g., "divided every 12 hours")
• Duration of therapy
• Indication for prescription
• Child's weight in kilograms

Treatment Duration Guidelines

• Most respiratory infections: 7-10 days 1, 2
• Pneumonia specifically: 10 days 1, 2
• Group A Streptococcal infections: 10 days (mandatory to prevent rheumatic fever) 2, 5, 4
• Minimum for all infections: Continue 48-72 hours beyond symptom resolution 4

Dosing Frequency: Why Twice Daily is Preferred

Twice-daily dosing (every 12 hours) improves adherence compared to three-times-daily regimens and is now the preferred approach 2, 3. The FDA label supports both regimens, but guidelines increasingly favor BID dosing for practical compliance reasons 1, 2, 4.

Monitoring and Follow-Up

• Clinical improvement should occur within 48-72 hours of starting treatment 1, 2, 5
• If no improvement by 72 hours: Reassess diagnosis and consider alternative antibiotics 2
• Complete the full prescribed course even if symptoms improve 1

Common Pitfalls to Avoid

Critical errors to prevent:

• Never use age-based dosing alone—always calculate based on weight 2, 3
• Never exceed 30 mg/kg/day in infants <3 months due to renal immaturity 4
• Never use 875 mg dose in severe renal impairment (GFR <30 mL/min) 4
• Never prescribe amoxicillin alone if MRSA is suspected—it will be ineffective 2
• Never forget to specify the indication on the prescription—this prevents pharmacy confusion 2, 3

Suspension Preparation and Storage

• Shake bottle vigorously before each use 4
• Refrigeration is preferable but not required 4
• Discard unused suspension after 14 days 4
• Administer directly on child's tongue or mix with formula, milk, juice, or water (must be taken immediately) 4

Adverse Effects to Monitor

• Most common: Gastrointestinal disturbances (diarrhea, nausea, vomiting) 1, 2
• Hypersensitivity reactions: Rash, urticaria 1, 2
• Monitor throughout treatment and counsel families on these potential effects 2