INTRODUCTION: There is an acknowledged care gap for patients with co-occurring substance use and mental ill-health. This study sought to use complexity theory to help make sense of the experiences of people who deliver or commission formal care for patients with alcohol use disorder and depression across one specific health and social care system.

METHOD: Qualitative interviews with 26 health and social care professionals in the North East and North Cumbria Integrated Care System, England, were conducted. Data analysis was undertaken using reflexive thematic analysis and informed by key concepts from complexity theory.

RESULTS: Three main themes were identified: (1) how the interplay between risk, stigma and resource pressures influences how care professionals interpret and apply practice guidelines; (2) how individualised and disjointed practices have structural and historical roots, in particular the impact of health service commissioning cycles; (3) ways in which practitioners have been able to adapt and engage in creative practice to temporarily plug gaps in care.

CONCLUSIONS: The pressure of working with increasingly scarce resources, within a highly fragmented, shifting, and risk-averse care infrastructure, adversely affected professionals' capacity to provide consistent, patient-centered support. Innovations have emerged that address some of these barriers, but further investment is needed to better support the substance use and mental health workforce, including lived experience peer workers.