Antibiotic exposure in the first year of life and later treated asthma, a population based birth cohort study of 143,000 children.

Several epidemiological studies reported an association between antibiotic consumption in the first year of life and later asthma, but results are conflicting and affected by potential biases. We examined this controversial issue in a population-based birth cohort. Using administrative data, we identified 143,163 children born in 1995-2011 in Friuli-Venezia Giulia (Italy) (median follow-up 5.25 years, 927,350 person-years). Antibiotic prescriptions in the first year of life and subsequent treated asthma (defined as ≥2 anti-asthmatic drug prescriptions within a 12-month period) were retrieved from drug prescription records. We estimated incidence rate ratios (IRR) using Poisson regression models, adjusted for perinatal variables and for hospitalizations for infections in the first year of life. We identified 34,957 new-onset asthma cases. Antibiotic consumption in the first year of life increased the risk of new-onset asthma [IRR 1.51, 95% confidence interval (CI) 1.48-1.54] with a dose-response relationship (p-trend <0.001). The risk was highest for asthma identified at 13-35 months of life (IRR 2.07, 95% CI 2.00-2.14), but remained statistically significant for asthma identified at 36-71 months (IRR 1.17, 95% CI 1.14-1.21) and at ≥72 months (IRR 1.15, 95% CI 1.08-1.22). Antibiotics increased the risk of current asthma at ≥6 years (IRR 1.35, 95% CI 1.30-1.41) and at ≥13 years of age (IRR 1.19, 95% CI 1.08-1.33). Antibiotic exposure in infancy is associated with an increased risk of asthma up to adolescence. The association detected at older ages is not explained by reverse causation; however, confounding by respiratory infections not leading to hospital admission cannot be excluded.