Executive summary

Surveys carried out in Australia have consistently indicated that cannabis is the most widely used illicit substance in the general population. It is therefore of considerable interest to public health to examine the patterns of association between cannabis use and the mental health and well-being of users.

There is limited clinical data on cannabis use among persons coming to the attention of mental health treatment services. Such clinical data is prone to selection biases. Some epidemiological research in the US has examined relationships between drug use disorders (of which cannabis use disorders were the most common) and other mental disorders, but there has been no examination of relationships between cannabis use and mental health in the Australian population.

The current report examined relationships between cannabis use and mental health using data from the Australian National Survey of Mental Health and Well-Being (NSMHWB). The NSMHWB provided nationally representative data on the mental and physical health of Australian persons aged 18 years and over. Unlike previous epidemiological surveys, the NSMHWB included measures of role functioning related to physical and emotional health, as well as measures of disability and psychological distress. Hence, the NSMHWB also provided an opportunity to examine the association between substance use and other indicators of psychological well-being.

The NSMHWB allowed the following questions to be answered:

is there an association between cannabis use, abuse and dependence, and other drug use and DSM-IV drug use disorders;
is there an association between cannabis use and DSM-IV affective and anxiety disorders, or with symptom measures of psychological distress and role functioning;
if any associations exist, are they explained by differences between cannabis users and non-users in demographic characteristics, levels of neuroticism, or in the levels of other drug use?

Around 1 in 20 persons (4.8%) reported using cannabis in the past year without meeting criteria for a DSM-IV use disorder. A further 0.8% met criteria for cannabis abuse, and 1.5% met criteria for cannabis dependence. In other words, around 300,000 adults in Australia met criteria for a DSM-IV cannabis use disorder within the past year, with another 650,000 using cannabis in a non-problematic fashion. All cannabis use was more common among males and those who were younger, with those aged 18-24 years most likely to report use, and meet criteria for abuse and dependence within the past year.

Those who reported using cannabis 5 times or less in the past 12 months (“non-users”) were least likely to report alcohol use or the use of sedatives, stimulants or opiates. As involvement with cannabis use increased, so too did the prevalence of other substance use. Regular tobacco use was much more likely among cannabis users, with 20% of non-users reporting such use, compared to 50-70% of persons with some involvement with cannabis use. Similarly, cannabis use of any sort was strongly related to an increased prevalence of alcohol use disorders, with more than one in 3 of those who met criteria for either cannabis abuse or dependence (37%) also meeting criteria for an alcohol use disorder. Other drug use disorders were similarly related, with very few non-users of cannabis meeting criteria for a use disorder (0.5%), compared to one in 6 cannabis-dependent persons having another drug use disorder (18%).

There was some attenuation of the relationships between cannabis use and mental health after accounting for the effects of demographics, neuroticism, and the use of other drug types. The association between cannabis involvement and all indicators of other substance use problems nonetheless remained highly significant. Regular tobacco use, alcohol use disorders, and other drug use disorders, were all related to involvement with cannabis independently of these other factors.

There was a strong univariate association between involvement with cannabis use in the past 12 months and the prevalence of affective and anxiety disorders. Among those with cannabis dependence, just over one in 7 met criteria for an affective disorder (14%), while 1 in 6 met criteria for an anxiety disorder (17%). In comparison, 6% of non-users met criteria for an affective disorder, and 5% met criteria for an anxiety disorder. Psychological distress and life satisfaction were also strongly related to involvement with cannabis on a univariate level. Those with increasingly higher involvement with cannabis reported greater levels of psychological distress (as measured by Kessler’s Psychological Distress scale), greater limitations in the everyday lives due to emotional distress (as measured by the SF-12), and lower life satisfaction.

The correlations between cannabis involvement and affective and anxiety disorders appeared to be explained by demographics, trait neuroticism, and other drug use. It must be said however that in including neuroticism as a predictor, we may be “overcontrolling” for this trait, since reported trait levels of neuroticism may be correlated with state levels of anxiety and depression. The data also do not exclude the possibility that an indirect relationship exists, e.g. if cannabis use increased the problematic use of other drugs, which in turn increased the risks of depression and anxiety. Future research might examine this issue further, particularly longitudinal research, and prospective studies examining the effect of reducing cannabis and other drug use on depression at a later point in time.

It is still the case that heavier involvement with cannabis use is correlated with affective and anxiety disorders. This has important clinical implications, since comorbid anxiety and affective disorders may affect the outcome of treatment for cannabis dependence. There is a lack of literature examining this possibility. Future work needs to examine the effect of other mental health problems upon treatment for cannabis dependence, as well as investigating more comprehensive therapies for persons wishing to address their cannabis use problem.

The high rate of comorbidity needs to be kept in mind by clinicians dealing with persons with cannabis use problems. Simple screening instruments could be used to examine whether a cannabis dependent client may have other mental health issues that need addressing. Future research might involve a more comprehensive intervention targeting symptoms of depression and anxiety as well as cannabis dependence.

In the Australian adult population, persons who used cannabis were more likely to screen positively for psychosis. Around one in 143 persons who were non-users screened positively, with the prevalence increasing as involvement with cannabis increased, such that one in 15 persons who met criteria for cannabis dependence also screened positively for psychosis. After controlling for demographics, neuroticism and other drug use, the relationship between dependent cannabis use and screening positively for psychosis relationship was still significant.

In the general population, many of those with cannabis use disorders will have other mental health problems, which is important given the increase in the numbers presenting to treatment agencies for help with their dependent cannabis use. Using the NSMHWB data, it was possible to calculate that in those who met criteria for cannabis dependence, two thirds (64%) with an affective disorder sought help, compared to one fifth (21%) of those without an affective disorder. Similarly, of those who met criteria for cannabis dependence and who also met criteria for an anxiety disorder, 80% sought help, compared to 16% of those without an anxiety disorder. This means that people coming to the attention of treatment services will be much more likely to have multiple problems; these need to be addressed in treatment.