The Implementation of Current UK Guidance for the Treatment of Co-Occurring Mental Health and Substance Use Conditions: A Mixed Methods Systematic Review and Thematic Synthesis.

Background: Clinical guidance exists on the provision of care for co-occurring mental health and substance use conditions in the UK. This systematic review aimed to identify the extent to which the guidance has been applied, including barriers and facilitators to delivery, and to examine use of strategies to support implementation of the guidance. Methods: We searched PsychINFO, MEDLINE, CINAHL, Embase, and Web of Science for articles published between 1st January 2017 (when the guidance was published) to 28th January 2025. Studies were eligible if they focused on service use and/or treatment provision for adults in the UK with co-occurring substance use and mental health conditions. The findings were analyzed using a thematic synthesis, through line-by-line coding of study findings to develop analytical themes. Results: Three themes were identified from 17 eligible studies. Theme one reflects "challenges to care for co-occurring conditions", which describes how service users face 'wrong doors'; issues surrounding the recognition of co-occurring conditions; and access to care when and where it is needed, despite guidance recommending the 'no wrong doors' approach. Theme two reflects approaches for the "integration of care" as recommended in the guidance, such as communication across services and ensuring that support for people with co-occurring conditions is 'everyone's job'. The third theme highlights barriers (e.g., stigma) and facilitators (e.g., therapeutic optimism) to applying the guidance. Conclusion: This review identified an implementation gap between best practice guidance and the provision of care for co-occurring conditions in the UK setting, which needs to be explored across a global context. There were positive examples of strategies to implement the guidance, particularly from small-scale intervention evaluations, yet studies of larger scale real-world intervention implementation are needed.