Comorbid mental disorders and substance use disorders: epidemiology, prevention and treatment

Impact of comorbidity on the course of illness and use of services

Page last updated: 2003

Comorbidity is a common problem in the general community, but it is even more common in patients presenting in primary care settings and most common in specialist services (Callaly, Trauer, Munro, & Whelan, 2001; Wittchen, 1996). Estimates of the prevalence of comorbidity in these samples are variable due to differences in such factors as diagnostic criteria (eg DSM-III-R versus DSM-IV) and time frames used. However, they all highlight the needs of comorbid groups and concomitant demands placed upon treatment provision. There is ample evidence from epidemiological surveys that treatment seeking is significantly increased where comorbidity is involved (e.g., Bijl & Ravelli, 2000; R. C. Kessler et al., 2001; Proudfoot & Teesson, 2002).

Common mental disorders
Type of drug

Common mental disorders

In reviewing the evidence from the National Comorbidity Survey in the United States, Kessler et al (1996) examined how the common mental disorders related to the course of comorbid substance disorder. They found that the presence of either a primary anxiety disorder or childhood conduct disorder and adult antisocial behaviour was associated with persistence of substance use disorders. In terms of exacerbation of symptoms, much of the literature is muddied by the lack of evidence regarding primacy of disorders as well as focusing on clinical samples which tend to not be representative of the relationship between comorbidity, illness course and service use.

A study carried out by Westermeyer et al; (1998) demonstrates the effects that having comorbid dysthymia has on substance abuse service usage. They identified those within a substance abuse treatment service who satisfied the criteria for an independent diagnosis of dysthymia and those who had a single substance use disorder. Of the 642 patients considered, only 39 were diagnosed with comorbid dysthymia. These were compared with those identified as having only a substance use disorder (N=308) in terms of their lifetime service use and the related costs. They found that those with comorbid dysthymia accessed psychiatric services no more than those with substance use disorders alone. Instead, the comorbid group accessed substance use treatment services more frequently and stayed in such treatment for longer periods than those with a substance use disorder alone. They estimated that, based on 1996 costs for treatment, those with comorbid dysthymia cost 4.7 times those with a substance use disorder only, in terms of substance use treatment dollars. Thus early detection and successful treatment of this disorder in individuals presenting at substance use services is likely to impact on future service usage and costs.

Recent evidence supporting the notion that the presence of substance use disorders makes the prognosis for other mental disorders worse comes from a study by Grant et al; (1996) based on the US Longitudinal Alcohol Epidemiologic Survey (NLAES). They found that those with comorbid alcohol and depression compared with those with lifetime major depressive disorder had a significantly earlier onset of major depression and were more likely to have more severe episodes of depression as measured by number of symptoms during their worst episode. They were also more likely to have a lifetime diagnosis of a drug use disorder.Top of page

Another study, by Hasin et al; (1996), also demonstrates the exacerbating effects of comorbid substance use disorders on affective disorders. They followed up 127 patients comorbid for alcohol dependence and major depression over five years and traced the patterns of remissions in both disorders. They found that irrespective of primary or secondary status, the risk of remission for depression was increased when alcohol dependence was also in remission. They did not find the converse, ie., remission of depression did not affect remission of alcohol symptoms. This suggests that the substance use disorder serves to maintain depression where remission is otherwise likely. The authors conclude that even where the depression exists independently of the substance abuse, it is likely that immediate treatment of the substance abuse can reduce depressive symptoms. They argue that future trials of interventions for depression and/or alcohol dependence should include comorbid patients and attempt to take aspects of both disorders into account.

Although the research is clear that there is an increased risk imposed by substance abuse on depression treatment outcomes, the evidence regarding the effects of depression on substance use treatment outcomes is equivocal (Lynskey, 1998). In his review of the literature, Lynskey states that comorbid females in alcohol abuse treatment have an increased risk of treatment failure, but the evidence regarding males is not conclusive. The Hasin (1996) study described above also suggests that there is no effect of the co-occurrence of a substance use disorder on remission for depression.

Thus the research suggests that treating substance use disorders in individuals with the more common mental disorders may improve their outcome and remission rates. However, treating these comorbid psychiatric disorders has not been shown to impact on substance use outcomes.


The psychoses include schizophrenia, schizo-affective disorder, bipolar disorder and depression with psychotic features. Individuals with substance use disorders who are suffering from these psychotic disorders are considered to have special needs due to the severity of their symptoms and the general disorganisation, both psychologically and socially, that these symptoms can cause.

For people with schizophrenia, substance use disorders are particularly problematic as they are generally directly associated with a range of negative outcomes. Much US research has found that, compared with people who suffer from mental illness alone, those with concurrent substance use show increased levels of medication non-compliance, psychosocial problems, depression, suicidal behaviour, rehospitalisation, homelessness, have poorer mental health and place a higher burden on their families (see Bartels, Drake, & McHugo, 1992; Clark, 1994; Drake & Wallach, 1989; Drake et al., 1990; Osher et al., 1994; Pristach & Smith, 1990). Persons with both types of disorders have also been recognised as being more difficult to treat than those with mental disorders alone (Drake, Mueser, Clark, & Wallach, 1996; Lehman, Herron, Schwartz, & Myers, 1993).

Because much of this data is from studies conducted in the United States it is important to consider the impact of comorbidity in countries with different health care systems. A study conducted in the United Kingdom by Menezes et al., (1996) on a geographic sample of patients with psychotic illness found the average number of admissions to psychiatric hospitals was similar for both those with illness alone and those who also abused substances. However, those who abused substances attended the psychiatric emergency service 1.3 times as often, and spent 1.8 times as many days in hospital, as those with mental illness alone.

In a recent Australian study, Hunt and co-workers (2002) analysed the effects of substance abuse on medication compliance and four year survival outcomes. They found that those who abused drugs over the period were significantly more likely to be re-admitted to hospital (median time to readmission 10 months) compared with those who did not abuse drugs (median 37 months). Even when patients were medication compliant, drug abuse tended to offset any advantages of this compliance. In another Australian study, Fowler et al., (1998) found that, for patients with schizophrenia identified in one geographical area, those with comorbid substance use problems tended to have increased rates of criminal behaviour, increased symptomatology and earlier age of onset of mental illness. However, they did not find the increased hospitalisation rates, suicide attempts and antipsychotic medication dosage reported elsewhere in the literature for this group. The disparity in findings regarding hospitalisation rates for these two Australian studies may be explained by the fact that the Fowler study had mobile teams available for extended hours to treat acute psychotic episodes in the home, which would have affected their hospitalisation rates. This model of treatment is based on the assertive community treatment approach to psychotic disorders which has been found to be superior to other models such as intensive case management (Issakidis, Sanderson,Teesson, Johnston, & Buhrich, 1999; Marshall & Lockwood, 2002; Rosen & Teesson, 2001). Even in a group as comorbid as the homeless, psychiatric outreach services based on assertive case management have been found to be effective in Australia (Buhrich & Teesson, 1996).
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Type of drug

Findings from clinical studies and population surveys suggest that alcohol and cannabis are the most common substances of abuse for people with psychotic disorders (e.g., Cuffel, Heithoff, & Lawson, 1993; Drake et al., 1990; Lehman, Myers, Dixon, & Johnson, 1994; Menezes et al., 1996). The study by Fowler and colleagues (1998) discussed above, found similar results in a sample of patients with schizophrenia attending a community mental health service in Australia. Apart from tobacco and caffeine, alcohol, cannabis and amphetamines were the most commonly abused substances. This contrasts with the contribution made by high rates of abuse of cocaine found in the US (Shaner et al., 1995).

Internationally, high use of stimulants such as amphetamines and cocaine has been implicated in increases in positive psychotic symptoms and earlier onset of symptoms in comorbid patients (Fowler et al., 1998; Shaner et al., 1995). In a review of the evidence regarding the relationship between cannabis and psychosis, Degenhardt and Hall (2002) conclude that there is little evidence that cannabis use per se causes psychosis. However, they state that it is likely that it exacerbates the illness and that it may precipitate it in vulnerable individuals. They point to the confounding effect of stimulant use amongst cannabis users in the various studies cited. The elevated levels of amphetamine use amongst cannabis users in Australia may explain increased psychotic symptoms in this group. The even higher prevalence of cocaine use in the US may also explain different findings regarding hospitalisations and suicidality between the US and Australia.

However, the literature is sparse and more research is needed to explain just how substance use affects the course of psychotic disorders and related service usage. Similarly, more research is required on the effects of the common mental disorders on substance abuse history. In one of the rare studies in this area, Westermeyer et al; (1998) found no significant effect for type of drug when investigating the effects of comorbid dysthymia on service use for those with substance use disorders.

Because comorbidity of mental disorders and substance abuse is common and has significant impact within the health care system and society, one important issue to consider is service delivery which may affect the incidence of these problems. The next section discusses the importance of prevention and early intervention.