Mental health consequences of cannabis use

I read with great interest the recent article by Pedersen (1). Cannabis is a widely used illegal drug worldwide. Very few studies have examined the impact of cannabis use on depression and suicidal behaviours (2) and hence the research question pursued in this study seems relevant. Strengths of the article also include a longitudinal design and control for a host of potentially confounding factors and reverse causality. A careful review of the article, however, reveals methodological limitations, which can be, overcome in future research in this area. Depression, an important outcome variable was measured by Kandel and Davies six item scale. None of these six items tap anhedonia, a cardinal symptom of depression. The measures also do not provide information about the presence of ICD-10 or DSM-IV lifetime and current depression and abuse and dependence to different substances. Future studies may therefore benefit from using structured diagnostic interview schedules which provide the above information. The authors have also not reported whether a proportion of the sample had received or were receiving treatment for depression. Further, the assessment of suicidal behaviours warrants a more comprehensive approach which elicits information about intentionality, lethality, inimicality and whether suicide attempts were made under the influence of a substance. Considering the above limitations in measurement, the conviction with which we can claim that early cannabis use predicted later suicidal behaviours but not depression is limited. Suicidal behaviours occurring in the absence of depression has been attributed to personality traits such as impulsivity (3) and hopelessness (4). Although the author attributed the above to impulsivity, this unfortunately was not supported by data. The number of respondents who met criteria for impulsivity and whether these respondents also endorsed suicidal behaviours was not reported. Both the neurophysiological and social process arguments provided by the author are not convincing enough. Future research therefore ought to replicate these findings and offer a better understanding of suicidal behaviours occurring in the absence of depression. Nevertheless, exploring mental health consequences of cannabis use has important implications for public health. Studies have demonstrated the effectiveness of cannabis derived substances in the treatment of medical illnesses, such as cancer and AIDS, resulting in demands to legalize the drug (5). Recent studies have also demonstrated the role of cannabis in psychotic and affective mental health outcomes (6). Riskbenefit assessment and policy decisions can only be made when adequate numbers of studies employing sound methodologies substantiate the above.