Remarkable advances in the management of individuals born with cystic fibrosis (CF) would not have been realized without empiric trial and error by CF clinicians with treatments developed and available for other purposes. As the testing and registration of CF-specific treatments have increased, so too have associated health care costs, particularly those of chronic medications. The transition of CF from a lethal pediatric disease to a life-shortening one with an adult majority, concurrent with sharp increases in chronic medication costs, has placed many CF treatments under increased scrutiny by third-party payers, particularly when prescribed to individuals from CF subpopulations that may not have been included in registration trials. Despite overall health improvements in the CF cohort and the increasing availability of CF-specific therapies, many physicians remain tasked with managing the health of patients from subpopulations that may be too young, too sick, or too complicated to have been included in clinical trials. An understanding of why particular CF subpopulations may have been excluded from registration trials, as well as consideration of a treatment's described mechanism of action, can support assessment for the potential for benefit (and risk) in these populations and help physicians advocate for patient access to treatments.