The association of harmful forms of illicit drug use with serious mental health problems is a key issue for national and international drug policy. The European Monitoring Centre for Drugs and Drug Addiction (EMCDDA) refers to ‘comorbidity/dual diagnosis’ as the ‘temporal coexistence of two or more psychiatric disorders as defined by the International Classification of Diseases, one of which is problematic substance use’ (EMCDDA, 2004). The relevance of the comorbidity of substance use and mental health disorders is related not only to its high prevalence but also to its difficult management and its association with poor outcomes for those affected. In comparison with patients with a single disorder, those with comorbid mental disorders and substance use disorders show a higher psychopathological severity (Langås et al., 2011; Stahler et al., 2009; Szerman et al., 2012) and increased rates of risky behaviour, which can lead to infection with diseases such as human immunodeficiency virus (HIV)/acquired immune deficiency syndrome (AIDS) and hepatitis C virus (Khalsa et al., 2008), psychosocial impairments (e.g. unemployment, homelessness) and criminal behaviour (Greenberg and Rosenheck, 2014; Krausz et al., 2013). Taking into account the burden on health and legal systems, psychiatric comorbidity among people with substance use disorders leads to high costs for society (DeLorenze et al., 2014; Whiteford et al., 2013).

Substance use and mental health disorders: a complex relationship

The relationship between substance use and mental health disorders is complex and it is difficult to establish a clear pathway between the two. The identification of psychiatric comorbidity is problematic, because the acute or chronic effects of substance abuse can mimic the symptoms of many other mental health disorders. Furthermore, a mental health disorder can have a negative impact on substance use (e.g. facilitate the start of substance use; increase the levels of drug use; facilitate risky patterns of drug use, etc.). Although convincing evidence supports a strong association between several mental health disorders and substance use disorders, the nature of this relationship may vary depending on the particular disorder (e.g. depression, psychosis, post-traumatic stress disorder) and the substance in question (e.g. alcohol, cannabis, opioids, stimulants).

Overall, the coexistence of two or more clinical conditions in the same individual raises two major clinical questions:

1. Is there an underlying common causal pathway?
2. What is the impact of this coexistence of clinical conditions on clinical care?

Pathways to comorbidity

Below we list four (non-exclusive) hypotheses that can be used to explain comorbidity.

1. The substance use disorder and another mental health disorder may represent two or more independent conditions.

In this case, the combination may occur through chance alone or as a consequence of the same predisposing factors (e.g. stress, personality, childhood environment, genetic influences) that affect the risk for multiple conditions. That is, substance use disorders and other psychiatric disorders would represent different symptomatic expressions of similar pre-existing neurobiological abnormalities (Brady and Sinha, 2005). Research in basic neuroscience has demonstrated the key roles of biological and genetic or epigenetic factors in an individual’s vulnerability to these disorders. But it has always to be considered that genes and neural bases are intimately interconnected with the environment.

2. The psychiatric disorder other than substance use disorder is a risk factor for drug use and the development of a comorbid substance use disorder.

In this scenario, different situations can be considered. In the ‘self-medication hypothesis’ (Khantzian, 1985), the substance use disorder develops as a result of attempts by the patient to deal with problems associated with the mental health disorder (e.g. social phobia, post-traumatic stress disorder, psychosis). In this case, the substance use disorder might become a long-term problem, or the excessive use of alcohol or an illicit drug might abate when the pre-existing mental health disorder is addressed appropriately (Bizzarri et al., 2009; Leeies et al., 2010; Smith and Randall, 2012). However, the psychiatric disorder could increase the risk of heavy and repetitive use of substances, leading to the development of a substance use disorder that might continue even when the pre-existing psychiatric condition is appropriately treated or remits (Moeller et al., 2001).

3. The substance use disorder could trigger the development of a psychiatric disorder in such a way that the additional disorder then runs an independent course.

Drug use can function as a trigger for an underlying long-term disorder. This is probably the most important mechanism underlying the association between cannabis use and schizophrenia. It is well known that cannabis use in vulnerable adolescents can facilitate the development of a psychosis that runs as an independent illness (Radhakrishnan et al., 2014).

4. The temporary psychiatric disorder is produced as a consequence of intoxication with, or withdrawal from, a specific type of substance, also called a substance-induced disorder.

Temporary psychiatric conditions (e.g. psychosis with features resembling schizophrenia) may be produced as a consequence of intoxication with specific types of substances (e.g. stimulants, such as amphetamines and cocaine) or withdrawal conditions (e.g. depressive syndromes associated with the cessation of stimulant use). The latest evidence of similar patterns of comorbidity and risk factors in individuals with substance-induced disorder and those with independent non-substance-induced psychiatric symptoms suggests that the two conditions may share underlying causal or aetiological factors (Blanco et al., 2012). Furthermore, there are some studies showing that, in some cases, previous induced disorders have been diagnosed as independent disorders after a follow-up period. These findings suggest that substance-induced disorders may be a transitory state prior to an independent disorder (Magidson et al., 2013; Martín-Santos et al., 2010).

Comorbidity and clinical care

Effectively responding to the coexistence of substance use and mental health disorders represents a challenge for treatment services in several ways. Among the issues that make treatment complicated are those surrounding the assessment of clients, the types of combination treatments they require, and the specific context and settings within which services are provided to them.

Clinical assessment

Clinical assessment with the detection and diagnosis of the two symptoms is a fundamental requirement to increase the possibility of positive treatment outcomes of people suffering from psychiatric comorbidity. However, the complex link between mental health and substance use disorders can make the clinical diagnosis of comorbidity difficult. Principally, this challenge revolves around distinguishing between the four causal or aetiological and neurobiological pathways into comorbidity discussed above. A number of instruments are used, with the main distinction being between screening and diagnostic instruments. Screening instruments are instruments that can be administered by an interviewer after a short training session, and which determine the initial needs for further follow-up on patients´ disorders or symptoms, and/or provide an early indication of psychiatric comorbidity. Diagnostic instruments are more in-depth and longer instruments, usually administered by expert professionals with a deeper knowledge of psychopathology. The choice of instrument will depend on the context (clinical, epidemiological, research), the assessment objectives (single or multiple diagnosis), the time available to conduct the assessment and the expertise of staff.

Specific combinations and clinical recommendations

The association between mental health and substance use disorders has different characteristics depending upon the specific combinations of the two disorders.

The specific clinical aspects of the more common combinations of psychiatric comorbidity include mood, anxiety, psychotic, attention deficit and hyperactivity, eating and personality disorders. Psychiatric comorbidity has an impact on the clinical severity, psychosocial functioning and quality of life of patients with substance use disorders. The therapeutic approach to tackle dual diagnosis, whether pharmacological, psychological or both, has to take into account both disorders from diagnosis in order to choose the best option for each individual. Optimal management requires a good understanding of the efficacy, interactions and side effects of pharmacological and psychological treatments. The clinical aspects and corresponding treatment recommendations of the most common combinations of different types of substance use and mental health disorders are presented next.

Jump directly to a combination:

• Depression + substance use disorders
• Anxiety + substance use disorders
• Psychosis + substance use disorders
• Personality + substance use disorders
• ADHD + substance use disorders
• Eating disorders + substance use disorders

Treatment services and implementation in European countries

Although specific clinical recommendations have been identified for most combinations of substance use and mental health disorder, there is broad agreement in the scientific literature that the two disorders should be addressed simultaneously and with a multidisciplinary approach, involving drug and mental health professionals working together towards common goals. However, there is still a lack of consensus regarding the most appropriate treatment setting and the most adequate pharmacological and psychosocial strategies. This has negative consequences for the patients, who encounter difficulties in identifying and accessing the best treatment for their disorders.

Three models of services have been implemented in Europe to date

1. The sequential model

In this model, the psychiatric and substance disorders are treated consecutively and there is little communication between services. Patients usually receive treatment for the most serious problems first, and, once this treatment is completed they are treated for their other problems. In this model the patient may be passed between services, increasing the risk of dropout and relapse from both services. For that reason evidence suggests that this model should be avoided when dealing with dual diagnosis patients.

2. The parallel model

In this model, treatment of the two different disorders is undertaken at the same time, with drug and mental health services liaising to provide services concurrently. Although some level of coordination between the two systems may be achieved, the two treatment needs are often met with different therapeutic approaches and the medical model of psychiatry may conflict with the psychosocial orientation of drug services. A potential negative consequence of this model is the lack of overall coherence of the treatment plan, which often falls on the patient.

3. The integrated model

In this model, treatment is provided within a psychiatric or drug treatment service or a special comorbidity programme or service. Cross-referral to other agencies is avoided. Treatments include motivational and behavioural interventions, relapse prevention, pharmacotherapy and social approaches. Although integrated treatment has been promoted as a way of reducing the fragmentation, duplication and risk of ‘falling between the gaps’ that may arise from sequential or parallel treatment models, the evidence supporting this is limited and is usually based on non-European approaches.

Conclusions

Psychiatric comorbidity is highly prevalent among substance users and is associated with increased levels of clinical and social severity. It has been associated with a poor prognosis of both psychiatric and substance use disorders and with fewer chances of recovery. At present there remains a lack of consensus regarding the most appropriate treatment settings and pharmacological and psychosocial strategies. This review notes that comorbid patients often have difficulties in accessing, and being coordinated within, required mental health and substance abuse services. The main barriers to the treatment are the separation of mental health and drug use treatment networks in most European countries, and the fact that treatment services may lack sufficient combined expertise to treat both types of disorders.