Background Despite extensive knowledge regarding the effect of pulmonary exacerbations treated with intravenous antibiotics on clinical outcomes in cystic fibrosis (CF), there is little known about the role of milder pulmonary exacerbations treated with oral antibiotics (oPEx).

Methods This was a retrospective cohort study of patients with CF followed at the Hospital for Sick Children and St. Michael's Hospital from 2009 to 2014. We evaluated the effect of oPEx on short-term clinical outcomes as the proportion of oPEx events in which 100% or 90% of baseline FEV₁% predicted was recovered at the end of treatment. We then examined the association of the number of oPEx events in the past 12 months on lung function (FEV₁% predicted) and nutritional status (body mass index (BMI) z-score) using a mixed-effects model.

Results There were a total of 2608 oPEx events in 570 subjects during the study period. In over half (53.4%) of oPEx events, lung function was already at 90% or higher of baseline FEV₁ at the initiation of oral antibiotic therapy and 82% were at 90% or higher of baseline FEV₁ at follow-up. In individuals with CF, one or more oPex events in the previous 12 months were associated with decreased FEV₁ compared with 12 months periods without oPex events. When the cumulative effect of oPExs on lung function was examined over the entire study period, patients with six or more oPEx events had the steepest rate of FEV₁ decline. oPEx events were not associated with changes in BMI.

Conclusions oPEx events are associated with short-term loss of FEV₁ and have a negative effect on lung function over time.