What is the issue
The co-occurrence of substance use disorders with other mental health conditions (dual disorders) is very common, and these patients typically have more severe problems – clinically and psychosocially – than patients with only substance use disorders.

The most frequent psychiatric comorbidities among individuals with substance use disorders are depression, anxiety, post-traumatic stress and personality disorders (mainly antisocial and borderline). The presence of these comorbid mental health problems increases emergency and psychiatric admissions, as well as the likelihood of relapse into drug use and premature death, and, overall, leads to poorer prognoses for both the psychiatric and the substance use disorders.

The emergence of the COVID-19 epidemic, both in relation to changes in the drugs used and the exacerbation of socio-economic stress factors, has raised additional concerns regarding the potential increased risk of other mental health comorbidities in people with drug use disorders.

Responding to comorbid substance use and mental health problems
There is broad agreement in the literature that these dual disorders should be addressed using a multidisciplinary approach in which drug use and the other mental issues are treated at the same time. However, there is a lack of consensus on the most appropriate treatment setting and the best pharmacological and psychosocial strategies to use.

The therapeutic approaches employed to tackle dual diagnosis, whether pharmacological, psychological or both, have to address both disorders from the first point of contact to identify the best option for each individual. A systematic approach is needed to detect and treat comorbid mental health problems in people with substance use disorders. Substance use and psychiatric disorders should be assessed using validated instruments. Standard screening instruments for both problematic substance use and psychiatric disorders can be used routinely when limited staff time or lack of expertise prevents more extended assessments.

In most European countries, the main barrier to treating comorbid substance use and psychiatric disorders is the separation of mental health and drug use treatment networks. This often means that each treatment service lacks sufficient expertise to treat both types of disorders, leading to a proliferation of different treatment approaches, regulations and financial resources.

Implications for the future
New models of care could facilitate a multi-disciplinary and multi-agency approach to addressing the complex needs of patients diagnosed with a drug use disorder and other comorbidities. Opportunities to improve models of care for these patients are likely to arise from the development of e- and m-health interventions that could potentially support recovery and social reintegration, and from the current development of more flexible and effective pharmacotherapies to respond to problematic drug use.

There is a need for:

• An in-depth review of service organisation in European countries, including a better understanding of the current barriers to interagency work.
• A multinational study using a standardised methodology to facilitate cross-national comparisons.
• The introduction into reporting systems across Europe of specific items about psychiatric comorbidity in substance use disorder patients, in order to allow routine monitoring.
• Treatment outcome studies to improve the evidence base for pharmacological and psychosocial therapies for people with comorbid substance use and psychiatric disorders.
• A comprehensive review of and research into possible early interventions to identify high-risk cases (e.g. young adolescents) in order to develop prevention measures.