There is mounting evidence of a link between antibiotic use in infancy and asthma in children, studies suggest.
Antibiotics are thought to kill off beneficial bacteria in the gut
A Canadian study of 12,082 children suggests those treated with antibiotics under the age of one are twice as likely to develop asthma in childhood.
And researchers writing in US journal Chest found additional courses of antibiotics in the first year of life increased asthma risk still further.
Earlier studies suggested the drugs may affect the way the immune system works.
Experts believe they kill off beneficial bacteria in the intestine and that this may lead to changes in the way the body deals with disease.
Lead study author Carlo Marra, of the University of British Columbia, Vancouver, said: "Antibiotic use in children has been found to coincide with an increased incidence of childhood asthma.
"Although the causal nature between antibiotics and asthma is still unclear, our overall results show that treatment with at least one antibiotic as an infant appears to be associated with the development of childhood asthma."
The Canadian team reviewed seven studies comparing exposure to at least one antibiotic to no exposure in the first year of life.
This analysis looked at 12,082 children and found 1,817 asthma cases were reported.
Overall, infants who were exposed to at least one antibiotic were twice as likely as unexposed infants to develop asthma during childhood.
The team also analysed data from five studies including 27,167 children looking at antibiotic doses.
It found that for each extra course of antibiotics during the first year of life a child was 1.16 times more likely to develop asthma.
Co-author Fawziah Marra said that, although antibiotics were commonly used to treat ear and respiratory infections and bronchitis, not every childhood infection needed antibiotics.
He said: "Current guidelines recommend that children under age two receive an antibiotic for diagnosed ear infection.
"However, the majority of upper respiratory tract infections and bronchitis are viral, for which antibiotics are ineffective."
Michael Alberts, president of the American College of Chest Physicians, pointed out that asthma was one of the most common chronic childhood diseases and affected millions of children in the US.
He added: "By identifying potential risk factors for asthma and educating patients and families about risk factors, we may begin to see a reduction in the overall incidence of asthma."
Dr Lyn Smurthwaite, research development manager at Asthma UK, welcomed the research into antibiotic use as a potential risk factor.
"Identifying events in early childhood that influence asthma development is a difficult task.
"The study shows that taking antibiotics in infancy may increase a child's risk of developing asthma and highlights that antibiotics should always be prescribed and taken responsibly," she added.