Research into the comorbidity (or co-occurrence) of mental disorders is a relatively recent phenomenon. Much of this research has been conducted in clinical samples, which are prone to a range of biases that mean that the patterns observed are not representative of the general population. Although population-level research has previously been carried out, this thesis provides the first examination of comorbidity in the Australian population. This thesis examined the comorbidity of substance use and mental disorders among Australian adults. The major empirical work involved an examination of the patterns of homotypic comorbidity (other substance use disorders) and heterotypic comorbidity (mood disorders, anxiety disorders, and psychosis) of alcohol, tobacco and cannabis in the 1997 Australian National Survey of Mental Health and Well-Being (NSMHWB). These drugs were chosen as they are the most commonly used psychoactive substances in the Australian population. The NSMHWB involved a structured diagnostic interview of mental disorders with a representative sample of Australian adults. Three questions were addressed using this data: (1) What patterns of comorbidity exist between tobacco, alcohol and cannabis use, and other substance use and mental disorders?; (2) Are these patterns of comorbidity explained by common factors?; and (3) Does comorbidity affect the likelihood that mental health treatment has been sought? Similar patterns of homotypic comorbidity were observed for all three substances, and they were not explained by the other factors examined (gender, age, education, relationship status, employment and neuroticism). Cannabis dependence was the most strongly associated with other substance use disorders. Heterotypic comorbidity differed between alcohol, tobacco and cannabis use. Tobacco use predicted increased rates of all three groups of mental disorders (mood, anxiety and psychotic disorders). In the case of alcohol, only alcohol dependence was related to increased rates of all groups of mental disorders; alcohol use and abuse were not associated with heterotypic comorbidity. Any level of cannabis involvement was related to a similarly increased risk of mood and anxiety disorders. Cannabis use was linearly related to the risk of screening positively for psychosis. Common factors did not change the patterns of heterotypic comorbidity of tobacco and alcohol use. However, alcohol, tobacco and other drug use appeared to explain the higher rates of mood and anxiety disorders among cannabis users. Treatment seeking was much more likely among alcohol, tobacco and cannabis users when they had comorbid mental disorders. It was moderately increased when they had comorbid substance use disorders. The second piece of empirical work provided a more detailed examination of comorbid substance use problems among persons with psychosis. This topic was selected due to the limited epidemiological research on this issue, and the relatively large burden of disability that psychosis places upon the individual and the community. NSMHWB data were used to examine the prevalence of comorbid substance use disorders among persons who were likely to have met criteria for psychosis (as assessed by a screener used in the NSMHWB). Multiple regression analyses were used to test possible explanations for the higher rates of substance use disorders observed among persons reporting higher numbers of psychotic symptoms. The odds of alcohol dependence and regular tobacco use increased 1.5 times, and the odds of cannabis dependence increased twice, with each additional psychotic symptom reported, after adjusting for other substance use disorders, other mental disorders and demographic characteristics. Given the debate about the reasons for the association between cannabis use and psychosis, the final study used mathematical modelling to test four hypotheses about relationships between cannabis use and psychosis. Specifically, it examined trends in psychosis that would be predicted given the marked increases in the prevalence of cannabis use that have occurred in Australia over the past thirty years. The results suggested that a causal relationship - in which cannabis use caused psychosis among persons who would not otherwise have developed the disorder - is unlikely to explain the association. There was a better fit to the data provided by the other hypotheses examined, namely, that (a) cannabis use precipitates psychosis among vulnerable individuals; (b) cannabis use increases the risk of relapse among persons with psychosis; and (c) persons with psychosis are more likely to become regular cannabis users (without any effect upon the disorder). This thesis has demonstrated that in Australian adults there is significant comorbidity between alcohol, tobacco and cannabis use and other substance use and mental disorders. These patterns differ across the three substances. Some types of heterotypic comorbidity (e.g. between cannabis use and mood/anxiety disorders) are explained by common factors. The limited range of common factors tested here did not explain homotypic comorbidity. This thesis also suggested that mathematical modelling is a useful approach to consider when examining the plausibility of different relationships between risk factors and mental disorders. A number of hypotheses regarding comorbidity could not be tested using NSMHWB data, such as common genetic and other environmental factors. These can best be tested in research with samples of twins, and using longitudinal designs that assess a wide range of social and environmental factors. The findings of this thesis also have implications for treatment, because persons with comorbid disorders are more likely to seek treatment. There is an absence of validated treatments for persons with comorbid substance use and mental disorders, and more research is needed on this issue.
| Identifer | oai:union.ndltd.org:ADTP/187789 |
| Date | January 2001 |
| Creators | Degenhardt, Louisa, Psychology, Faculty of Science, UNSW |
| Publisher | Awarded by:University of New South Wales. School of Psychology |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | Copyright Louisa Degenhardt, http://unsworks.unsw.edu.au/copyright |
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