NDARC Technical Report No. 78 (1999)
The National Survey of Mental Health and Well-Being (NSMHWB) interviewed a representative sample of 10,641 Australians aged 18 years and older about symptoms of DSM-IV and ICD-10 mental health and substance use disorders, and disability and help-seeking associated with these disorders. It provided the first opportunity to examine the prevalence of cannabis use disorders in the adult Australian population.
This report presents data on the prevalence of cannabis use, the prevalence and correlates of DSM-IV cannabis use disorders, the DSM-IV dependence symptoms reported and health service utilisation among Australian adults. A limited comparison with ICD-10 cannabis use disorders is presented. The factor structures of DSM-IV and ICD-10 dependence symptoms and agreement between the two diagnostic systems in who is diagnosed with a disorder are also examined.
In the past 12 months, 7.1% of Australians had consumed cannabis on more than five occasions; it was the most commonly used illicit drug among Australians. Cannabis use predominated among males (10.1% vs 4.2% of females) and 18-24 year olds (19.8%, compared to 12% or less in older age groups).
During this time 2.3% of Australian adults, and 31.7% of cannabis users, were diagnosed with a DSM-IV cannabis use disorder. This was predominantly cannabis dependence (1.5% of the population and 21% of users) as opposed to abuse (0.8% of the population and 10.7% of users). Cannabis use disorders were the second most common diagnosed substance use disorder. As with use, disorders tended to predominate among males and young adults.
The most commonly reported DSM-IV dependence symptoms were: a persistent desire, or unsuccessful efforts to moderate use (36.6%) and withdrawal symptoms (29.7%). Users were typically only mildly dependent, meeting a mean of 1.3 out of the 7 dependence criteria.
A multivariate analysis of level of cannabis involvement (no cannabis use, non-dependent use, DSM-IV dependent use) identified a number of correlates of increased involvement. Demographic factors were: being male, being younger, being unemployed, not being in a married/defacto relationship and being Australian born (compared to being born in a non-English speaking country). Correlates indicative of a comorbidity between cannabis dependence and substance use or mental health factors were: being a current tobacco smoker, having a DSM-IV alcohol use disorder, illicit drug use involvement, and having higher neuroticism scores on the EPQ.
Those who met DSM-IV cannabis dependence criteria were approximately 3 times as likely as those without to have sought help from a health professional in the past 12 months for a mental health problem than those without. These figures were on a par with those diagnosed with alcohol dependence. However, given a number of factors were associated with level of cannabis involvement, the effect of these on the relationship between dependence and health service utilisation needs to be investigated.
As a comparison 1.7% of Australian adults and 23.6% of users were diagnosed with ICD-10 cannabis use disorders in the past 12 months. As with the DSM-IV diagnoses, dependence was more common than harmful use, and disorders were more common among males and younger adults. The most commonly reported ICD-10 dependence symptoms were: difficulties controlling cannabis use (43.2%) and withdrawal symptoms (29.7%). Again, users clustered at the mild end of the dependence continuum, meeting an average of 1.3 out the 6 ICD-10 criteria.
Principal components analyses (PCA) of DSM-IV and ICD-10 dependence symptoms indicated that among the general Australian population, cannabis dependence symptoms formed a unidimensional syndrome with good internal consistency.
There was excellent agreement between the DSM-IV and ICD-10 diagnostic systems as to who was diagnosed with cannabis dependence (kappa=0.9) and severity of dependence (r=0.9). Agreement between the systems on the proportion of Australians with a cannabis use disorder was substantial, but slightly lower (kappa=0.7), largely because of the poor agreement between the DSM-IV abuse and ICD-10 harmful use diagnoses.
These results show that cannabis use disorders are the second most common form of substance use disorder in the Australian population, affecting approximately 300,000 adults. The significance of cannabis dependence needs further investigation, particularly its clinical relevance and its causal significance in the likelihood of developing adverse health outcomes. Cross-sectional surveys such as these thus need to be supplemented by longitudinal research and intervention studies.