The US Epidemiological Catchment Area study (ECA) was the first large-scale epidemiological study to collect information on comorbidity in the community. The study was conducted between 1980 and 1984 on approximately 20,000 respondents aged 18 years and older in the US. Among those respondents with a lifetime alcohol use disorder (alcohol abuse or dependence) 37% had at least one other mental disorder and 22% had another drug disorder (Regier et al., 1990). Highest levels of comorbidity were found for those with a lifetime history of any drug disorder other than alcohol; with 53% with comorbid mental disorders and 47% with comorbid alcohol disorders. Conversely, among individuals with any lifetime mental disorder, 29% had some addictive disorder — 22% had a lifetime history of an alcohol use disorder and 15% had a lifetime history of a drug use disorder. Highest rates of substance use disorders were found amongst those with an antisocial personality disorder (84%) and schizophrenia (47%). Lesser but still substantial rates of substance abuse disorders were also found amongst those with affective disorders (32%) and anxiety disorders (24%).

The more recent National Comorbidity Survey (NCS) was undertaken between 1990 and 1992 to examine the extent of comorbidity between substance use and non-substance use disorders in the US population (Kessler et al., 1994).The NCS was conducted on approximately 8,000 respondents aged between 15 and 54 years. This study measured both lifetime and current prevalences of mental disorders. Lifetime comorbidities were found to be comparable to those found in the ECA. The prevalence of any current (12 month) diagnosis was 29.5% (Kessler et al., 1994). Among those with a 12 month diagnosis of any substance use disorder, 36% had at least one anxiety disorder whilst 25% had at least one affective disorder. Among those with a 12 month diagnosis of any mental disorder, 15% had at least one co-occurring substance use disorder (Kessler et al., 1996).

Australian National Survey of Mental Health and Wellbeing (NSMHWB)

In 1997, the Australian Bureau of Statistics was funded by the Australian Government to undertake the NSMHWB, a survey of the mental health of the Australian adult population. Until this survey Australia was reliant on US studies for epidemiological data on mental health, substance use disorders and comorbidity. The survey provides the first national Australian data on the prevalence and patterns of mental disorders among Australian men and women, highlighting the extent of comorbidity in the general population. It was conducted on a nationally representative sample of 10,641 Australians and designed to answer three main questions: How many Australians have which mental disorders? How disabled are they by these disorders and what services have they used for these disorders? A modified version of the Composite International Diagnostic Interview (CIDI, WHO, 1997) identified the most common mental disorders (namely, anxiety, affective and substance use disorders) using the two major psychiatric classification systems, DSM-IV and ICD-10.The survey achieved a high response rate — 78% of eligible adults aged 18 and over, completed the survey interview and very few who agreed to begin the interview withdrew. Results of the survey were weighted to ensure that the estimates were representative of the total adult population (Henderson, Andrews, & Hall, 2000).Top of page

In the Australian mental health survey, about 18% of the respondents (aged 18–90 years) met criteria for a DSM-IV mental disorder in the past 12 months. This is somewhat lower than the NCS figure of 29.5%. This difference may be due to the age groups covered (younger ages in the NCS); the omission of certain disorders from the NSMHWB (in particular Antisocial Personality Disorder and Specific Phobias); and the use of DSM-IV diagnoses in the NSMHWB and DSM-III-R in the NCS.

Results of the survey show a considerable degree of comorbidity in substance use disorders and other mental health (Teesson, Hall, Lynskey, & Degenhardt, 2000). About one in four persons with an anxiety, affective or substance use disorder also had at least one other mental disorder. This meant that they had two or more different classes of disorder, such as an anxiety and affective disorder, or an anxiety and a substance use disorder. A small proportion of men (0.8%) and women (0.8%) had all three types of disorder (ie. an anxiety, affective and substance use disorder) (Andrews, Hall,Teesson, & Henderson, 1999).

Figure 1 and Figure 2 summarise the prevalence and comorbidity data found in the survey. Among those individuals with mental disorders, marginally more women than men had at least one other comorbid mental disorder (28% of women, as against 24% of men with any of these mental disorders). The patterns of comorbidity differed between men and women reflecting the differences in prevalence within the sexes for the individual disorders. Among women, affective and anxiety disorders most often occurred together, accounting for three quarters of women who had more than one mental disorder. Among men, comorbid disorders more often involved an anxiety or an affective disorder in combination with a substance use disorder. These combinations of disorders affected two thirds of men who had more than one mental disorder.

The National Survey of Mental Health and Wellbeing included a low prevalence study of psychotic disorders. This study demonstrated that drug and alcohol use disorders were highly prevalent in those with psychotic illnesses. Nicotine was the most commonly used drug in this sample with 67% using nicotine in the previous 12 months. Lifetime diagnoses of alcohol use disorder were found in 30% of the sample and cannabis use disorder in 25%. Jablensky and co-workers (2000) also looked at a sample of people with psychoses who were marginalised or homeless and not seen by mainstream services. They found more than double the levels of comorbid substance abuse, which contributes to the intensification of psychotic symptoms found in this group.

The high rates of comorbidity have a number of implications for treatment and management. Mental disorders complicated by alcohol and other drug use disorders, and vice versa, have been recognised as having a poorer prognosis than those without such comorbid disorders. They are also more likely to become chronic and disabling, and result in greater service utilisation.

Figure 1: Prevalence (%) of single and comorbid affective, anxiety and substance use disorders amongst Australian males in the past year

Text equivalent below for Figure 1: Prevalence (%) of single and comorbid affective, anxiety and substance use disorders amongst Australian males in the past year

Text version of Figure 1

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  • Prevalence of single affective, anxiety and substance use disorders amongst Australian males in the past year:
    • Substance use disorders - 8.3%
    • Anxiety disorders - 3.6%
    • Affective disorders - 1.4%

  • Prevalence of comorbid affective, anxiety and substance use disorders amongst Australian males in the past year:
    • Substance use and anxiety disorders - 1.4%
    • Substance use and affective disorders - 0.6%
    • Anxiety and affective disorders - 1.3%
    • Substance use, anxiety and affective disorders - 0.8%

Figure 2: Prevalence (%) of single and comorbid affective, anxiety and substance use disorders amongst Australian females in the past year

Text equivalent below for Figure 2: Prevalence (%) of single and comorbid affective, anxiety and substance use disorders amongst Australian females in the past year

Text version of Figure 2

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  • Prevalence of single affective, anxiety and substance use disorders amongst Australian females in the past year:
    • Substance use disorders - 2.4%
    • Anxiety disorders - 7.3%
    • Affective disorders - 3.6%

  • Prevalence (%) of comorbid affective, anxiety and substance use disorders amongst Australian females in the past year:
    • Substance use and anxiety disorders - 0.9%
    • Substance use and affective disorders - 0.3%
    • Anxiety and affective disorders - 3.1%
    • Substance use, anxiety and affective disorders - 0.8%