However, the investigators note that they cannot exclude the possibility that their findings were based on reverse causation or confounded by indication, in which the presence of asthma resulted in more frequent respiratory tract infections, which in turn increased the rate of antibiotic use.

The prevalence of asthma in western countries has increased over the last 3 decades, Dr. Carlo A. Marra and colleagues at the University of British Columbia in Vancouver note, which seems to coincide with greater exposure of infants to antibiotics. Nonetheless, epidemiologic evidence linking antibiotic use with asthma risk is conflicting.

To conduct a meta-analysis, Dr. Marra's group identified seven studies that specifically examined the relationship between receipt of at least one prescription for an antibiotic in the first year of life and the development of physician-diagnosed asthma between the ages of 1 and 18 years.

According to their report in the March issue of Chest, the study cohorts included 12,082 children and 1817 asthma cases.

Overall, the pooled odds ratio was 2.05 for asthma risk associated with antibiotic exposure before 1 year of age. The association was significantly stronger in the five retrospective studies (OR = 2.82) than in the prospective studies (OR = 1.12).

Dr. Marra's group also pooled data from five trials analyzing a potential dose-response relationship, which included 27,167 children and 3,392 asthma cases.

The OR was 1.16 for each additional course of antibiotics taken during the first year of life. Again, there was a trend toward a stronger relationship in the two retrospective studies (OR = 1.37) than in the three prospective studies (OR = 1.07).

The authors suggest that the methodologic concerns they encountered -- including reverse causation, site of infection, and type of antibiotics used -- will need to be overcome in further large-scale, database-related studies to definitively answer whether or not antibiotic use early in life is associated with later asthma risk.

Meanwhile, it is possible to safely reduce the number of antibiotics that infants receive, co-author Dr. Fawziah Marra notes in a press release. Although antibiotics are commonly used to treat upper respiratory tract infections and bronchitis, she notes that most of these infections are viral - for which antibiotics are ineffective.

Chest 2006;129:610-618.