Health reporter, BBC News
Antibiotic resistance is an increasing problem
Patients may have to be prescribed higher doses of antibiotics because of rising rates of obesity, say doctors.
The standard "one-size fits all" dose may not clear infection in larger adults and increases the risk that resistance will develop, they argue.
More work is needed to guide GPs on how and when to alter doses, an editorial in The Lancet to accompany the study by doctors from Greece and the US says.
GPs said it was an interesting theory but may end up being expensive.
Around one in four adults in England is classified as obese - an increase from 15% in 1993.
Given the fact people are getting larger, use of standard doses of antibiotics in all adults, regardless of size, is outdated, argue two doctors from Greece and the US.
Size and even the proportion of body fat a person has, can affect the concentration of antibiotics in the body, potentially reducing how effective they are in larger patients, they say.
And failure to clear an infection because too small a dose is given may raise the risk of resistance - already an increasing problem for doctors.
Likewise, smaller than average patients may get too much drug, and suffer greater side-effects as a consequence.
An accompanying editorial said dose adjustments could easily be made if research was done to guide doctors in treating obese patients.
Professor Steve Field, chair of the Royal College of GPs said he would encourage "appropriate" antibiotic prescribing and lots of patients are given them unnecessarily.
But he added: "Patients are getting taller and larger and it does seem right that patients are given the appropriate strength of drug.
"However, this might cost a lot of money because pharmaceutical companies would have to provide different doses of medication.
"At the moment, most come in two strengths and we would not want to see an increase in costs."
He added that GPs will already use their judgment to alter medication doses where necessary.
Professor Hugh Pennington, an expert in antibiotics from the University of Aberdeen, said antibiotics would differ in how size altered their effectiveness.
"But studies on this would not be hard to do.
"If you have too little of a drug it's not going to be good for treating the infection but it also raises the possibility that the organism will become resistant.
"They're such powerful drugs, we want to make sure we are using them properly."