Children: Dosing

Last updated: Thursday, July 09, 2015

This is a common question. If the medicine is licensed for the relevant age group for the right indication then good starting points are the Children’s BNF (BNF-C), Neonatal Formulary, or SPC. Sometimes SPC advice may differ from your paediatric text, so always check both.

However, the majority of enquiries relate to unlicensed/‘off-label’ use and so SPC information is less likely to be helpful (see Legal Considerations). If the medicine is frequently used in children ‘off-label’ (e.g. sildenafil), then try established paediatric sources first. For newer medicines or those used more rarely (e.g. clopidogrel), you may have to undertake a literature search to see if any authors have published their previous experience.

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Failing that, contact the manufacturer to see if they have any unpublished data, or speak to experts in the field such as your hospital’s paediatric pharmacist, your nearest children’s hospital, or national paediatric specialist hospitals such as Great Ormond Street (London) or Alder Hey (Liverpool).

If there simply is no information, then proceed with extreme caution. Every scenario will be slightly different and you’ll need to have a frank discussion with the enquirer, exploring alternative medicines or routes, or whether the drug is needed at all.

Extrapolation from adult doses is generally not recommended due to the differences in drug handling in children, but in exceptional cases may be considered.

Practical Tip

When advising on a dose, also think about the practicalities of how the medicine will be given: can your dose actually be measured accurately? Consider whether the dose can be rounded up or down to make administration easier (and safer) depending upon the therapeutic range.

For example a 20-month-old child weighing 14.2 kg starting omeprazole at 700 micrograms/kg once daily would require a dose of 9.94 mg. Because the drug does not reliably suspend in water and omeprazole has a wide theraputic range, it is safer to round up to the nearest whole tablet (10mg).

Remember that for most drugs the adult maximum dose should not be exceeded. For example, an older child weighing 70kg prescribed cefotaxime 50mg/kg every 6 hours for severe meningitis should be given the maximum adult dose of 12 grams per day, rather than 14 grams that the calculation would predict.

Calculating drug doses for children who are obese is a separate consideration, and you’ll need to go back to first principles as you would in an obese adult. For example:

  • What is the volume of distribution of the drug? Does it stay in the vascular compartment or distribute into fat?

  • What is the therapeutic range of the drug? The more narrow, the more cautious you are likely to be.

  • What is the side effect profile of the drug? The greater the risk of serious toxicity, the more careful you’ll need to be.