Challenges in minimizing the adverse effects of cannabis use after legalization.

Abstract

Over the past 20 years epidemiological studies have left us better informed about the adverse effects of regular cannabis use in young adulthood on mental health and psychosocial outcomes [1]. The Christchurch Health and Development Study (CHDS) of David Fergusson and colleagues has made an especially valuable contribution to this literature, as indicated by the summary of its key findings on the mental health and psychosocial outcomes of cannabis use in the current issue. The CHDS followed the life course of a birth cohort of 1000 New Zealanders, 80 % of whom had used cannabis by their mid-20s. Nearly a third used cannabis regularly enough, and for long enough, to enable assessment of associations between common adverse psychosocial and mental health outcomes and regular cannabis use. The study also collected detailed information on the personal and social situations of the cohort that enabled the researchers to assess whether the associations they observed between daily cannabis use and adverse outcomes could be explained by pre-existing differences between those who had and had not used cannabis regularly. This study has consistently found associations between daily cannabis use and poor mental health and psychosocial outcomes. Daily cannabis users had poorer educational attainment in adolescence and poorer employment outcomes in young adulthood, were more likely to use other illicit drugs, and were more likely to report more symptoms of psychosis, depression and suicide. Many of these risks increased with the intensity of cannabis use, and these associations persisted after statistical adjustment for plausible confounding factors. Many of these results have been replicated in the Dunedin birth cohort (e.g. [2]), and in longitudinal studies in other countries such as Australia (e.g. [3]), Germany [4] and the Netherlands [5]. Fergusson and colleagues show that the adverse health effects of cannabis are most concentrated among daily users (nearly 20 % of those who ever used the drug). This pattern was most common among young people who began using cannabis in their mid-teens and continued to use daily throughout young adulthood. Not all cannabis users experienced harm: many adolescent users did not use cannabis regularly enough or for long enough to do so; and a substantial proportion of adult cannabis users, the majority of whom reported using cannabis less than daily, did not report any harms related to their cannabis use. The cannabis policy debate has been presented in many countries as a forced choice between two packages of belief: (1) that cannabis use does not harm users, and so it should be legalized to avoid users acquiring criminal records; and (2) that cannabis use can harm some users, and so we should continue to prohibit its use [1]. This framing has often hindered a fair appraisal of the adverse health effects of cannabis. Evidence on the harms of cannabis use cannot decide cannabis policy [6]. The policy choice will depend upon societal beliefs about the priority that should be given to competing social values, such as, preserving individual freedom and protecting human health and well-being. It will also depend on political views on the extent to which the state should use criminal law to prevent adults from & Wayne Hall w.hall@uq.edu.au