Penicillin prescriptions for millions of children over the age of five could be below the recommended doses.

A collaborative study conducted by Imperial College London, King's College London and St George’s, University of London found that 40 per cent of 6-12 year olds and 70 per cent of 12-18 year olds are prescribed doses below current national guidelines.

If penicillin doses are below those needed to effectively treat bacterial infections they could result in infections that get worse in some children, increase the demand on health services and increase bacterial resistance to treatment.

Published in British Journal of General Practice, the study is the first to examine actual prescriptions of oral penicillins by GPs in the UK. It reported on the most commonly prescribed penicillin, amoxicillin, and compared actual doses to the doses recommended by the British National Formulary for Children (BNFC).

Analysing data from the IMS Disease-Analyzer database, researchers found there was a wide variation in prescribing practices. None of the children under the age of one year received  BNFC's recommended dose of 62.5 mg per unit. In fact, the majority of children under the age of one received  double this recommended dose (125 mg).

In comparison, 96 per cent of children aged 1-5 received the recommended dose for their age group but the picture was different for older children. 40 per cent of 6-12 year olds and 70 per cent of 12-18 year olds were prescribed unit-doses below the BNFC guidelines. Similar patterns of prescribing were also found for penicillin V and flucloxacillin.

The first author of the study, Dr Sonia Saxena from Imperial's School of Public Health and a GP said: "It's not clear why there is such widespread under dosing. It may be that GPs are unaware of the recommended doses or that we err on the side of caution in our prescriptions for fear of overdosing. Alternatively some GPs may aim to limit possible side effects such as diarrhoea.

"However there are significant harms in under dosing children with penicillin. Firstly, bacterial infections can become resistant and fight back rendering any treatment ineffective in the community.  Although the majority of children will still get better, using too low a dose can sometimes result in some children getting more serious infections.For example if the dose of penicillin to treat tonsillitis or a serious ear infection is too low this can reduce the effectiveness of the treatment so these children may get worse and either have to return for another GP appointment or in some cases could require treatment or even admission to hospital."

The BNFC guidelines currently recommend dosing oral penicillins in children accordingly to age-bands, weight-bands or weight-based calculations but many UK GPs routinely use only age-bands. In 2013 there was a change to the guidelines specifically for amoxicillin but the researchers suggest this may in fact have worsened the situation because it introduced a wider age-band from 5 to 18 years. This is likely to increase the  under dosing for older ages from 12 to 17 years in whom the pre 2013 dose was equivalent to an adult dose of 500mg. Yet 70 % received half this dose

"The guidance needs to be urgently reviewed and recalculated so we move away from an age-based system to a weight-based system," said Dr Saxena. "The ideal situation would be for us to actually weigh children and then prescribe doses accordingly but we know there are difficulties with this including GPs being short of time and introducing a more  complex system could actually result in more prescribing errors. However we can consider other options such as providing scales in the waiting room or encouraging pharmacists to double-check the dose against the child's actual weight when people collect prescriptions."

Even if weight-bands from the BNFC guidelines were used, the researchers point out another problem with the current guidelines: that the average weight of children has changed over time and the guidelines do not take this into account. Due to the rising prevalence of childhood obesity, prescriptions could lead to doses prescribed at too low a level to produce a therapeutic effect.

Dr Sonia Saxena is funded by a National Institute for Health Research (NIHR) Career Development Fellowship.

Reference: Saxena et al. Comparison of UK oral penicillin prescribing for children with British National Formulary for Children age-band recommendations: a descriptive analysis. British Journal of General Practice, April 2014