Formulation is considered a fundamental process of cognitive behavioural therapy for psychosis (CBTp). However, an exploration into the personal impact of different levels of case formulation (CF) from a service user (SU) perspective is lacking, particularly for those experiencing a first episode of psychosis. This Big Q qualitative design used semi-structured interviews. Reflexive thematic analysis (TA) was used to analyse 10 participant interviews. NVivo 12 computer-assisted qualitative data analysis software aided data organisation and analysis. One overarching theme 'CF - A vehicle for change?' was developed as a pattern of shared meaning across the data set. Three main themes related to the overarching theme: (1) Vicious cycles: 'I never really thought about it being me maintaining the problems' (including one subtheme - Self-empowerment: 'Only you can make the changes for yourself'); (2) Early life experiences: 'My experiences have shaped the person that I am, therefore, it's not my fault' (including one subtheme - Disempowerment: '[My] core beliefs have been damaged'); and (3) Keep it simple: 'Don't push it too far over the top in case it becomes like spaghetti'. Maintenance formulations may be experienced as self-blaming, but also self-empowering, which may help to facilitate change. Longitudinal formulations may be experienced as non-blaming, but also disempowering, which may inhibit change. Simple CF diagrams may also facilitate change, whereas overly complex CFs may inhibit change. How CBTp therapists might look to improve the impact of different levels of CF for service users (SUs) in first episode psychosis (FEP) are described.