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The coexistence of substance use disorders (SUD) and other mental disorders - commonly referred to as dual disorders (DD) - is highly prevalent and clinically significant. Although various terms have been used over time (e.g., psychiatric comorbidity, dual pathology), the lack of a standardised definition has created inconsistencies in diagnosis, communication, research, and treatment. Epidemiological studies estimate that 30-80% of individuals with SUD have co-occurring psychiatric disorders, with prevalence depending on the substance involved, gender, age, and the population studied (general population, people in emergency rooms, mental health addiction facilities, harm reduction facilities, prison). Compared to individuals with a single disorder, people with DD exhibit more severe psychopathological symptoms, higher suicide risk, increased relapse rates, poorer treatment adherence, more medical comorbidities, and worse psychosocial outcomes, including unemployment, homelessness, and social exclusion. Despite extensive evidence, both SUD and other mental disorders often remain underdiagnosed and undertreated, partly due to diagnostic complexities such as overlapping symptomatology and the syndromic nature of psychiatric classifications. Additional barriers include the structural separation between mental health and addiction services and limited integrated care expertise. The "wrong door syndrome" exemplifies how patients are frequently directed to services unable to address both conditions, leading to suboptimal outcomes. Furthermore, individuals with DD are often excluded from clinical research, limiting the evidence base for tailored interventions. Improving the detection and treatment of DD is a major challenge for mental health and addiction systems. Integrated, holistic treatment approaches from the outset are essential to optimise outcomes for this highly vulnerable population.