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Irene Mommers,1 Job FM van Boven,2,3 Catharina CM Schuiling-Veninga,1 Jens HJ Bos,1 Marten Koetsier,1 Eelko Hak,1 Maarten J Bijlsma1,4 1Pharmacotherapy, Epidemiology and -Economics, University of Groningen, Groningen, the Netherlands; 2Groningen Research Institute for Asthma and COPD, University Medical Center Groningen, Groningen, the Netherlands; 3Department of Clinical Pharmacy and Pharmacology, University Medical Center Groningen, Groningen, the Netherlands; 4Laboratory of Population Health, Max Planck Institute for Demographic Research, Rostock, GermanyCorrespondence: Irene Mommers, University of Groningen, Department of Pharmacotherapy, -Epidemiology and -Economics, Antonius Deusinglaan 1, Building 3214, Room 0450, Groningen, 9713 AV, the Netherlands, Tel +31 50 36 32476, Email i.mommers@rug.nlPurpose: The Global Initiative for Asthma (GINA) suggests a step-wise approach for pharmacological treatment of asthma. Valid study of real-world treatment patterns using dispensing databases includes proper measurement of medication adherence. We aim to explore such patterns by applying a time-varying proportion of days covered (tPDC)-based algorithm.Patients and Methods: We designed a retrospective inception cohort study using the University of Groningen IADB.nl community pharmacy dispensing database. Included were 19,184 young adults who initiated asthma medication anywhere between 1994 and 2021, in the Netherlands. Main treatment steps were defined as: 1 - SABA/ICS-formoterol as needed, 2 - low dose ICS, 3 - low dose ICS + LABA or tiotropium, or intermediate dose ICS, 4 - intermediate to high dose ICS + LABA or tiotropium, triple therapy, or high dose ICS, 5 - treatment prescribed by a specialist. Changes in treatment steps were determined using a time-varying proportion of days covered (tPDC)-based algorithm. Individual drug treatment trajectories were visualized over time using a lasagna plot.Results: At initiation, of the 19,184 included individuals, 52%, 7%, 15%, 16%, and 10% started treatment in steps 1 to 5, respectively. The median (IQR) follow-up time was 3 (1– 7) years. Median (IQR) number of switches was 1 (0– 3). Comparing starting step to last observed step, 37% never switched between treatment steps, 20% of individuals stepped down and 22% stepped up.Conclusion: The low proportion of treatment switches between steps indicates that tailoring of treatment to patients’ needs might be suboptimal. The tPDC-based algorithm functions well in translating dispensing data into continuous drug-utilization data, enabling a more granular assessment of treatment patterns among asthma patients.Keywords: asthma management, respiratory disease, inhaler medication, personalized medicine, longitudinal, observational