Objectives: To explore one aspect of the external validity of the randomized controlled trial (RCT), specifically how being selected for inclusion in a trial and having participated has influenced the use and cost of asthma-related health services.
Methods: Services used by asthmatic users of inhaled corticosteroids (iCSTs) having previously participated in an RCT (TS, n = 46) were compared with individuals who had never participated (NS, n = 51).
Results: TS were more likely to use higher (≥400 μg) daily doses of iCSTs than NS (OR, 3.3; 95% CI, 1.1–8.3) but less likely to visit emergency departments (OR, 0.3; 95% CI, 0.1–0.7). Total asthma-related costs did not differ significantly.
Conclusions: Subject differences may impede generalizing from RCTs to real life.