Background Cystic fibrosis (CF) is a multisystemic disease caused by mutations in the cystic fibrosis transmembrane conductance regulator ( CFTR) gene, resulting in defective synthesis or function of the CFTR protein. Historically, CF treatment focused on managing symptoms and complications. Fortunately, modulator drugs are now available to directly target the defective CFTR protein. However, in some countries, such as Turkey, these drugs are not covered by social insurance. Consequently, many CF patients face barriers to accessing modulatory therapies or must interrupt their treatment. This study demonstrates the impact of interrupting modulator therapy on pulmonary function, emphasizing the need for uninterrupted continuous treatment. Methods In this study, 39 CF patients receiving elexacaftor-tezacaftor-ivacaftor (ETI) at our clinic were retrospectively analyzed. Among the patients, 18 experienced one or more interruptions, ranging from 15 to 210 days during ETI treatment. We analyzed pulmonary function test results from 27 interruption periods. Results At the beginning of the interruption, the mean percent predicted FEV1 (ppFEV1) was 69.59 ± 25.87%, which decreased to 64.96 ± 24.52% by the end of the interruption. There was a significant decrease with a mean change of 4.62 ± 8.49 (p = 0.008). However, no significant correlation was found between the interruption duration and FEV1 change. Conclusion Our results demonstrate that pulmonary functions are adversely affected by interruption periods, regardless of their duration. Even short interruptions have a significant impact on pulmonary functions. This underscores the need for uninterrupted continuation of modulatory treatment and for improved policies to ensure equitable access to treatment.