Almost five million children who are prescribed penicillin each year for infections are getting too little of the antibiotic to be effective, putting them at risk of serious complications.
The most common penicillin prescription is for otitis media, an infection of the middle ear which can lead to deafness if not properly treated. Infections of the urinary tract are also common in children.
Scientists say dosing guidelines for penicillin have remained unchanged for 50 years while the average weight of children has increased. As a result the amount of drug given per kilogram of body weight has fallen which "could potentially lead to failed treatment."
Giving too little of the drug to destroy the bacteria causing the infection also contributes to the development of antibiotic resistance which poses a growing worldwide threat. Infections resistant to almost all known antibiotics are increasingly rendering existing drugs useless.
Researchers at King's College London and St George's, University of London, say recommendations for prescribing penicillin by age were first introduced in 1963, accompanied by average weights.
The dose recommended for a five year-old was based on an average weight of 18kgs and for a 10 year-old on an average weight of 30kgs.
But the Health Survey for England 2009 showed the weight of a five year-old was 21kgs and the average weight of a 10 year-old was 37kgs.
Writing in the British Medical Journal, the authors say dosing guidelines for adults have been "substantially increased" over the decades to reflect rising average weights and in response to increasing fears of antibiotic resistance. There has been no rise in children's dosing.
They say doses seem "strikingly low" at older ages. A 10 year-old child weighing 40kgs who was prescribed 250mg of amoxicillin three times a day would receive less than half the recommended dose per kilogram of body weight.