Page 40 - A Guide to Dosage Form 1

The dose for each medication are usually determined for adults and undeniably, paediatric will
require a smaller dose as compared to the adults. Ideally, this should also be determined
through clinical research studies but this is not always possible.

Hence, methods were developed on how to calculate the dosage in the most accurate
manner. For some drugs that have a narrow therapeutic window (narrow range between
desired therapeutic efficacy and severe toxicity) such as anticancer drugs, dosage calculation
should be done very carefully.
Usually, doses are calculated by using either the body weight or the body surface area.
If the body weight is to use, the dose will usually be expressed in mg/kg. Example;

The dose of paracetamol is 15 mg/kg/dose, three times a day

The child’s weight is 10 kg

Thus, the dose for the child will be = 15 mg x 10 kg
= 150 mg for each dose, three times a day

If the dose in mg/kg is not available, Clark’s Rule can be used to calculate the children
dose from a known adult dose.

Clark’s Rule

ℎ ℎ ℎ
Paediatric dose =
75
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The second method of calculation is by using the body surface area. This is a more
accurate method and is used especially when extreme accuracy is needed.
Dose calculation based on body surface area

Body surface area (m2) = Weight (kg) 0.425 x Height (cm) 0.725 x 0.007184

( ) ℎ ℎ ( 2)
Paediatric dose =
1.73 2

* 1.73 m2 is the body surface area of an average adult
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Apart from these two calculation methods, paediatric doses can also be calculated
based on a child’s age by using the formulas below:

Fried’s Rule for Infants
( ℎ)
Paediatric dose =
150