Doctors believe the drugs can alter the way the metabolism functions, making it more likely that some people will pile on the pounds.
In a study of mice, scientists found those given antibiotics gained up to 15 per cent more fat after just six weeks than animals that did not receive them.
Dr Ilseung Cho, of New York University, said even low doses of the commonly used drugs may alter the composition and function of bacteria in the gut known as microbiome.
He said: “By using antibiotics, we found we can manipulate the population of bacteria and alter how they metabolise certain nutrients. Ultimately, we were able to affect body composition and development in young mice by changing their gut microbiome through this exposure.”
The expert said scientists are only now beginning to understand just how complex the bacterium is and how it is likely to be linked to someone’s health.
Dr Cho’s team decided to carry out the study after learning how farmers have used antibiotics to fatten livestock, including cattle and sheep, for decades.
Their tests showed weight gain, which was all or mostly fat, of between 10 and 15 per cent in mice after just six weeks of taking the drugs.
The other rodents, which followed the same diet but were not given the drugs, did not gain weight, the study in the journal Nature showed.
Study leader Professor Martin Blaser said: “This work shows the importance of the early life microbiome in conditions like obesity. The rise of obesity around the world is coincident with widespread antibiotic use, and our studies provide an experimental linkage.
“We are still learning how far the impact of the microbiome reaches and the costs of perturbing it.”
The US experts hope their study will lead to further research on the bacterium’s link to metabolism in a bid to find new weight-loss and fat-fighting treatments.
Antibiotics are among the most frequently prescribed medications in the UK and are given to patients to kill off harmful bacteria in the body.
Patients can become resistant to the drugs and in recent years many GPs have attempted to cut the number of antibiotics they prescribe.
Scotland has the world’s second highest obesity rates in the developed world behind the United States and one in five Scots is now obese.
A different US study has found that babies given antibiotics before they were six months old appeared to increase their chance of being overweight by the time they were aged three by almost a quarter.
Scientists who carried out the research believe giving antibiotics at such a young age could damage the babies’ developing digestive systems.
Their study, published in the International Journal of Obesity, found bacteria in the intestines seemed to play critical roles in how people absorb calories and exposure to antibiotics could be killing off healthy bacteria that influence how we absorb nutrients and keep people lean.
Dr Leonardo Trasande, a paediatrician at the New York University School of Medicine, said: “We typically consider obesity an epidemic grounded in unhealthy diet and exercise, yet increasingly studies suggest it’s more complicated.”