Alcohol, tobacco and cannabis are among the most commonly used psychoactive substances in the Western world. One issue that has received increasing attention in recent years concerns the comorbidity (or co-occurrence) of the use of these substances with other substance use and mental health problems. However, there appears to have been no comparison of the associations of these drug types with a range of mental health problems in the general population.

This study compared patterns of comorbidity of alcohol, cannabis and tobacco with other mental health problems, and with other drug use and drug use disorders. This was examined using data from the Australian National Survey of Mental Health and Well-Being (NSMHWB). The NSMHWB provided nationally representative data on Australians aged 18 years and over.

The following questions were asked:

1. What are the relationships between the level of involvement with use of alcohol, tobacco and cannabis, and:
   • other drug use and DSM-IV drug use disorders;
   • DSM-IV affective disorders;
   • DSM-IV anxiety disorders;
   • screening positively for psychosis?
2. What are the relative strengths of these associations?
3. Are these associations explained by demographic differences between users and non-users of cannabis, alcohol and tobacco?
4. Are these associations explained by different levels of other drug use between users and non-users of cannabis, alcohol and tobacco?
5. Are these associations explained by differences in neuroticism between users and non-users of cannabis, alcohol and tobacco?

In this general population sample, the strongest markers of other drug use were alcohol dependence and any involvement with cannabis use, with cannabis dependence being the strongest marker overall. Cannabis dependence was also the strongest marker of meeting criteria for another drug use disorder. In contrast, the strongest marker of anxiety and affective disorders was alcohol dependence. These findings did not appear to be explained by a number of other factors considered here. Cannabis dependence remained a significant marker of screening positively for psychosis. Tobacco smoking was a consistent marker of poorer mental health, remaining associated with higher rates of affective and anxiety disorders, substance use disorders, and screening positively for psychosis.

These findings suggest that different drug types are differentially associated with different patterns of comorbidity. These differential risks need to be taken into account in treatment. Further research is required to further examine these findings.