Paediatric Weight Based Dosing

Weight based dosing scales the dose to body mass so that drug exposure is comparable across a very wide range of sizes, expressed as mg/kg for each dose or for each day. It is the default method up to about 40 kg, after which the adult dose is usually used.

Which weight. Use a weight measured on the day. If it cannot be measured, estimate and replace the estimate as soon as possible. Infants under 1 year: (age in months divided by 2) plus 4 kg. Children 1 to 10 years: (age in years plus 4) multiplied by 2 kg. A length-based tape is safer in emergencies.

Dose and daily total. Confirm whether a figure is mg/kg for each dose or mg/kg/day divided over several doses. Confusing the two is a common cause of 3 to 4 fold errors.

Representative weight based doses for common situations:

Medicine	Dose	Maximum
Paracetamol oral	15 mg/kg every 4 to 6 hours	60 mg/kg/day and 4 g/day
Ibuprofen oral	5 to 10 mg/kg every 6 to 8 hours	30 mg/kg/day and 1.2 g/day; not under 3 months
Amoxicillin oral (pneumonia)	40 to 45 mg/kg every 12 hours	3 g/day
Adrenaline intramuscular (anaphylaxis)	10 mcg/kg (0.01 mL/kg of 1 mg/mL), repeat after 5 minutes if needed	500 mcg each dose
Dexamethasone oral (croup)	0.15 to 0.6 mg/kg single dose	10 mg
Glucose 10% intravenous (hypoglycaemia)	2 mL/kg	Repeat after 10 minutes if glucose remains low

Weight band dosing. Artemether-lumefantrine (20 mg/120 mg tablet) for uncomplicated malaria is given as six doses over 3 days (0, 8, 24, 36, 48 and 60 hours): 5 kg to under 15 kg, 1 tablet; 15 kg to under 25 kg, 2 tablets; 25 kg to under 35 kg, 3 tablets; 35 kg and over, 4 tablets. Take with fat-containing food.

Obesity. For hydrophilic drugs (gentamicin, vancomycin loading) use adjusted weight: ideal weight plus 0.4 times the excess over ideal. Ideal weight is the weight at the 50th centile of body mass index for age and height. Cap every dose at the adult maximum.

Malnutrition. Use current weight for most drugs, accept reduced hepatic and renal clearance, and avoid fixed combination products that cannot be split accurately.

Neonates. Dose by gestation and postnatal age as well as weight, because clearance depends on maturity.

Rounding. Round to a volume the syringe can measure, within 10% of the calculated dose, and never round up for narrow-margin drugs.

Special groups. Chronic therapy needs dose review as the child grows, since fixed doses slowly fall in mg/kg terms. Dosing tapes and age-band charts are acceptable in emergencies, with measured weight replacing them once available.

Monitoring. Recalculate when weight changes by more than 10%. Check the calculation twice for opioids, insulin, potassium, digoxin, aminoglycosides and anticonvulsants, and confirm the concentration of the product (mg in how many mL) before drawing up.

Referral and escalation. A calculated dose over the adult maximum, any ten-fold decimal doubt, or a weight that does not fit the child’s age and appearance needs pharmacist or senior confirmation before administration.

Stopping. Record the stop date with each weight based course and re-weigh before any repeat course.

Paediatric Weight Based Dosing

Principles And Patient Context

Medicine Selection And Regimens

Dose Adjustment And Special Situations

Monitoring, Escalation And Stopping