BACKGROUND: Recent studies revealed that some people with cystic fibrosis (pwCF) receiving the full dose of Elexacaftor/Tezacaftor/Ivacaftor (ETI) experience adverse events (AEs), leading to discontinuation or dose adjustments of their modulator treatment. We aimed to investigate if pwCF who experienced AEs had high exposure to ETI, and to what extent dose reduction resulted in changes in exposure, side effects and efficacy. METHOD: This retrospective case series presents ETI exposure in pwCF who required ETI dose reduction due to AEs. ETI exposure was measured by minimal concentration (C RESULTS: The mean C CONCLUSION: This study suggest that AEs of ETI in most cases could be related to elevated exposure levels. Dose reduction decreases ETI exposure and AEs while maintaining efficacy as determined with sweat chloride levels.