What dose of intravenous paracetamol (10 mg/mL) should be administered to a 5‑month‑old infant weighing 5.6 kg?

Intravenous Paracetamol Dosing for 5-Month-Old Infant (5.6 kg)

For a 5-month-old infant weighing 5.6 kg, administer 7.5 mg/kg of intravenous paracetamol every 6 hours, which equals 42 mg (4.2 mL of the 10 mg/mL solution) per dose, with a maximum total daily dose of 30 mg/kg/day (168 mg or 16.8 mL total per 24 hours).

Dosing Calculation

Based on the most recent pharmacokinetic evidence and safety data:

• Single dose: 7.5 mg/kg × 5.6 kg = 42 mg per dose
• Volume to administer: 42 mg ÷ 10 mg/mL = 4.2 mL per dose
• Frequency: Every 6 hours (4 doses per day)
• Maximum daily dose: 30 mg/kg/day × 5.6 kg = 168 mg per day (16.8 mL total)

Evidence-Based Rationale

The dosing recommendation is supported by multiple pharmacokinetic studies demonstrating that:

• Target therapeutic concentration for analgesia is 10-20 mg/L (or 9-11 mg/L mean steady state) 1, 2, 3
• Age-specific clearance: At 5 months of age, paracetamol clearance is still developing. Clearance increases from birth with a half-life of 3.25 months, reaching approximately 60-70% of adult values by this age 2
• Licensed dosing for infants: The 7.5 mg/kg dose every 6 hours (total 30 mg/kg/day) is the licensed regimen for term neonates and infants under 1 year in Europe 4

Administration Guidelines

Infusion parameters:

• Administer each dose over 15 minutes as an IV infusion
• Do not give as a rapid bolus
• Can be given undiluted or diluted in normal saline or dextrose solutions

Timing:

• Maintain strict 6-hour intervals between doses
• Do not exceed 4 doses in 24 hours

Important Safety Considerations

Critical warnings for this age group:

1. Reduced hepatic conjugation: Infants have immature glucuronide conjugation but enhanced sulfation pathways, making them relatively protected from hepatotoxicity at appropriate doses 5

2. Weight-based dosing is essential: Patient size (weight) accounts for 57.5% of clearance variance in neonates and infants 1. Never use fixed dosing.

3. Monitor for cumulative toxicity: While single-dose studies are common, data on repeated dosing safety in young infants is limited 5. Avoid exceeding the 30 mg/kg/day maximum.

4. Contraindications to verify:

   • Hepatic impairment
   • Severe renal impairment
   • Known hypersensitivity to paracetamol

Common Pitfalls to Avoid

• Do not use adult dosing formulas: The 10-15 mg/kg dosing used in older children would result in supratherapeutic levels and potential toxicity in infants under 6 months
• Do not round up aggressively: At this low weight, even small rounding errors can lead to significant overdosing
• Do not use rectal formulations interchangeably: Rectal bioavailability is different and more erratic in young infants 2, 5
• Do not assume faster clearance: Unlike older children who may require higher mg/kg doses, infants have slower clearance and require lower doses 2

Duration of Therapy

• Reassess need for continued IV paracetamol after 48-72 hours
• Consider transition to oral/rectal formulations when appropriate (though bioavailability differs)
• Maximum recommended duration of IV therapy is typically 5-7 days without hepatic function monitoring

This conservative dosing approach prioritizes safety while achieving therapeutic analgesic concentrations, based on the most robust pharmacokinetic data available for this vulnerable age group 1, 2, 3.