Background Clinicians’ treatment beliefs could affect the feasibility of delivering different treatments in a randomised controlled trial (RCT). In MAPS (Managing Adolescent first Episode Psychosis: a feasibility Study), adolescents with first episode psychosis (FEP) were randomly allocated to receive either antipsychotic medication (AP), psychological intervention (cognitive behavioural therapy [CBT] and family intervention [FI]), or both. We conducted a nested qualitative study to investigate clinicians’ views of these treatments. Methods Purposive sampling identified seventeen clinicians from CAMHS and Early Intervention services with prescribing responsibilities for 14-18 year olds at three participating MAPS sites. Individual participants were interviewed to examine their views of treatments in the MAPS trial. Interview transcripts were analysed using inductive Thematic Analysis. Findings Clinicians viewed the decision to refer adolescents to the MAPS trial as requiring careful clinical judgement. Assessment complexity and diagnostic uncertainty had to be balanced against the urgency for treatment to reduce risk and distress. Underlying influences including duty of care and treatment beliefs underpinned decisions. Clinicians consistently valued AP as the primary treatment for FEP, with CBT and/or FI seen as helpful secondary treatment options. Nevertheless, the potential harms of prescribing AP, or not, to such a young population were highlighted as being of concern in treatment decision-making, and fostered reluctance to refer into a RCT. Interpretation The design and delivery of RCTs involving young people experiencing FEP should consider the views of responsible clinicians, recognising that perceived treatment urgency, limitations in diagnostic precision, and existing treatment beliefs may influence trial processes. Funding NIHR HTA programme (project number 15/31/04).