Natacha Biset,1,* Mélanie Lelubre,1–3,* Christelle Senterre,4 Karim Amighi,1 Olivier Bugnon,2,3 Marie P Schneider,2,3 Carine De Vriese1 1Department of Pharmacotherapy and Pharmaceutics, Faculté de Pharmacie, Université libre de Bruxelles (ULB), Brussels, Belgium; 2School of Pharmaceutical Sciences, University of Geneva, University of Lausanne, Geneva, Switzerland; 3Community Pharmacy, Department of Ambulatory Care & Community Medicine, University of Lausanne, Lausanne, Switzerland; 4Research Center of Epidemiology, Biostatistics and Clinical Research, School of Public Health, Université libre de Bruxelles (ULB), Brussels, Belgium *These authors contributed equally to this work Purpose: The aims of the study were to evaluate medication adherence and the influencing factors for isotretinoin and contraception (oral, patches, and rings) and to evaluate the concomitant use of contraception and isotretinoin. Methods: Reimbursed prescription data from January 2012 to August 2015 of all patients in Belgium were received from Pharmanet–National Institute for Health and Disability Insurance. Medication adherence was measured according to the medication possession ratio. The influence of gender and age was analyzed using the Mann–Whitney test and the Spearman coefficient correlation. The independence between adherence to contraception and adherence to isotretinoin was analyzed using the Pearson chi-square test of independence. Persistence was defined as the number of days between initiation and presumed end of treatment. The Kaplan–Meier method was used to plot the medication persistence curves, and the log-rank test was used to compare the curves. The concomitant use of contraception and isotretinoin was analyzed using descriptive statistics. Results: The medication possession ratio was ≥0.8 for 46.1% of patients receiving isotretinoin and for 74.0% of women using contraception. For isotretinoin, this percentage decreased as the number of attempts increased (29.8% for the second attempt and 19.8% for more than two ...