Penicillin doses for children need to be reviewed to take account of the fact youngsters are getting heavier, experts have said.
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.
Currently, the average weight of a five-year-old is 21kg and a 37kg for a 10-year-old - up to 20% 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 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 equals half an adult, small child equals half a big child, baby equals half a small child."