PENICILLIN doses for children need to be reviewed to take account of the fact youngsters are getting heavier, experts say.
Dosing guidelines have remained unchanged for almost 50 years and are mostly based on children's ages.
But experts argue that the dose a child needs is determined by their weight -- and the average weight of children has increased over the last 50 years.
The average weight today of a five-year-old is 21kg and 37kg for a 10-year-old -- up to 20pc higher than in 1963, they said.
Many children will actually not be receiving a large enough dose, they added.
Low dosing could also potentially drive resistance to antibiotics "with consequences for both the individual and the community".
The UK-based researchers said: "Underdosing may result in the need for retreatment and increases the risk of severe complications.
"All the published risk-benefit analyses on the therapeutic balance of antibiotic prescribing for upper respiratory tract infections assume adequate antibiotic dosing.
"This is a real concern because clinically inadequate dosing would increase the number needed to treat to prevent any severe complications."
The study, led by a team at King's College London and St George's, University of London, said they were "surprised at the lack of recent evidence" to support current dosing recommendations for penicillins.
Writing in the British Medical Journal, they added: "The widely used doses of 62.5mg or 125mg are fractions of the adult dose recommended in the British National Formulary and are still based on the original dosing principle of a big child = half an adult, small child = half a big child, baby = half a small child."
The team analysed the actual dose that would be received today based on age bands recommended in the 2010/11 British National Formulary for Children and the current weights of children based on 2009 Health Survey for England data.
The results showed doses could be strikingly low.
The authors also pointed out that many infections do not need treatment with antibiotics.
"Many of the five million children in England who receive oral penicillins each year may not need them, but those who do should receive them in an effective dose."
Dr Paul Long, senior lecturer in pharmacognosy at King's College London, said: "Children's average size and weight are slowly but significantly changing, so what may have been adequate doses of penicillin 50 years ago are potentially not enough today."