Researchers conducted a study in which they investigated a link between antibiotic use in children younger than one year and development of asthma. The odds for developing asthma by age 7 were 1.86 for children who received antibiotics for non-respiratory infections in their first year. For those children given multiple antibiotic courses and broad-spectrum antibiotics, the odds were significantly higher.

The researchers studied a birth cohort of 13,116 children born in 1995 in Manitoba, using the province's health care and prescription databases. More than half of the study participants lived in urban areas, one-quarter came from low-income families, 5% had a mother with asthma, and nearly all had siblings (90%). Six percent had asthma at age 7. Sixty-five percent received at least one antibiotic in their first year of life, three percent received narrow-spectrum antibiotics, fifty-two percent received broad-spectrum antibiotics while ten percent of children received both.
The antibiotics were prescribed for conditions such as otitis media (for 40% of the children), upper respiratory tract infection for 28%, and lower respiratory tract infection for 19%. Seven percent of the children received antibiotics for non-respiratory infections.

The study found that asthma was significantly more likely to occur at age 7 in children who had received antibiotics in the first year of life for non-respiratory tract infections than in those who had not received antibiotics at all; risks of developing asthma increased in a dose-dependent manner and was significant for every number of antibiotic courses; the asthma risk for children who received more than four courses of antibiotics in the first year was even greater for those without a dog in the home at birth, with no maternal history of asthma and those who lived in a rural area. Both broad-spectrum and narrow-spectrum antibiotics were associated with the development of asthma by age 7 although broad-spectrum antibiotics carried more risk.

Atopic dermatitis was found to be a major risk factor for the development of asthma. The non-respiratory infections for which children received antibiotics were skin infections. It is suspected that early life skin infections are caused by an impaired barrier function of the epithelium that may lead to allergen penetration and subsequent inflammation.

The researchers stated that the study could not confirm a causal role of antibiotics in children developing asthma. This is why large-scale studies are needed to further examine these associations.