Background <p>Co-occurring substance use disorders, and mental disorders are associated with significant suffering, high risks of violent reoffending and relapse in both substance use and psychotic episodes. Treatment for co-occurring substance use disorders in forensic mental health settings would benefit from incorporating patients’ understanding of their disorders.</p> Method <p>Using an interpretative phenomenological analysis, this study explored and interpreted the experiences of 13 Swedish forensic psychiatric patients regarding the development of, and being treated for, co-occurring disorders.</p> Results <p>The analysis yielded four experiential themes: “substituting a missing tool”, “’it’: a self-sustaining ecosystem”, “treated but not cared for”, “comprehending fragments of the self”. Results illustrated how substance use, mental and physical health and social worlds fused into an interconnected ecosystem (referred to as “it”) where negative developments in any domain risked inducing relapse. Diagnosis-focused treatment was perceived as disconnected from this reality, leading to selective censorship and treatment disengagement. In understanding their past, patients experienced a fragmentation between their identities resulting in the conceptualisation of an “addict self” who inhabited “it” and who was separate from their current, “moral self”.</p> Conclusion <p>Flexible treatment approaches that address early-life trauma and that assist in integrating patients’ past actions, present vulnerabilities, and future potential are beneficial to the forensic mental health services.</p> Clinical trial number <p>Not applicable.</p>