General study design
The RCT with a parallel design assessing the effect of multiple-dose OT administration in children with ASD was performed at the Leuven University Hospital (Belgium). The double-blind phase (phase I) was followed by a 4-week single-blind phase (phase II) during which all participants received intranasal OT. In both phases, OT administration effects were assessed immediately after the 4-week administration period (i.e., post-measurement T1 and T3) and at a follow-up session 4 weeks after cessation of the daily administrations (i.e., follow-up measurement T2 and T4). Please see Fig. Trial design (panel Written informed consent from the parents and assent from the child were obtained prior to the study. Consent forms and study design were approved by the Ethics Committee for Biomedical Research at the University of Leuven, KU Leuven (S61358) in accordance with The Code of Ethics of the World Medical Association (Declaration of Helsinki). The trial was registered at the European Clinical Trial Registry (EudraCT 2018-000769-35) and the Belgian Federal Agency for Medicines and Health products. As indicated in the EudraCT registration, behavioral data collections were part of a broader assessment, including (neuro)physiological and biological assessments (reports in preparation). The trial was monitored by the Clinical Trial Center at the University Hospital of Leuven, and all trial staff had Good Clinical Practice certification and was trained in the study protocol.