Accessible Summary What is known on this subject? There is a need to shed light on healthcare professionals’ reactions to the use of the Guided Self‐Determination method in a mental health hospital to better understand and adjust the implementation process of evidence‐based practice. Healthcare professionals´ values and beliefs play an important role when implementing evidence‐based practice in real‐world healthcare settings. What this paper adds to existing knowledge? The study identifies opposite positions in mental healthcare professionals: being ready or resistant to change when implementing an evidence‐based intervention. The positions are elaborated in four thematic dynamic continuums describing reactions to using the intervention. In addition, this is the first study to explore mental healthcare professionals´ reactions to using the Guided Self‐Determination method in a mental health context. What are the implications for practice? When preparing implementation of an evidence‐based intervention, it is important to consider adaptation of the intervention, the mental healthcare professionals’ acceptability, support from management and participation in supervision. In future research, it is important to consider collecting data from mental healthcare professionals trained in using an evidence‐based intervention, however not using it in clinical practice, to understand barriers towards evidence‐based practice. Abstract IntroductionEvidence‐based interventions are required in mental health nursing to improve quality and outcome for patients. However, there is a need to shed light on professionals’ reactions to the use of evidence‐based interventions to better understand and adjust the implementation process. AimTo explore mental healthcare professionals´ reactions to using the evidence‐based intervention Guided Self‐Determination method in the care of inpatients with severe mental illness. MethodA qualitative study conducted in relation to an 8 months implementation program. Data collection: 9 qualitative interviews and field notes generated from supervision of the intervention. ResultsFour themes emerged from a thematic analysis: “The expert becomes novice,” “Theory used as a looking glass,” “Guided Self‐Determination perceived as an interruption” and “Becoming an informer of the impact of GSD.” DiscussionUsing the themes may help leaders or researchers predict or discover the support needed by individual professionals. Implications for practiceWhen preparing implementation of an evidence‐based intervention, it is important to consider adaptation and acceptability, as well as support from management and participation in supervision. Finally, it is worth to consider collecting data from trained professionals, who did not use the intervention in practice to understand barriers towards evidence‐based practise.