Abstract
The majority of New Zealanders (approximately 80%) born in the 1970s report using cannabis at least once, despite its illegal status. Four-10% of past-year users were at risk of developing a cannabis dependence syndrome that impaired psychological, social and/or occupational functioning. There were negative psychiatric consequences for a subset of the population who began using cannabis in early/mid-adolescence, particularly in terms of developing psychosis (the risk appeared to be highest for those with a genetic predisposition), and to a lesser degree for depression. There was a consistent dose-response relation between increasing levels of, and/or persistence of cannabis use and a range of deleterious health outcomes including loss of cognitive capacity, increased respiratory symptoms and impaired lung function, periodontal disease, compromised educational achievement and employment history, as well as a host of negative social outcomes (e.g. criminal convictions, relationship difficulties, driving impairment). No discernable impacts upon cardiovascular function were observed. The majority of cannabis users did so with little or no harm. In contrast, a non-trivial minority of the population (approximately 5%-10%) were at heightened risk because they: (i) used cannabis on more days than not; (ii) had become cannabis dependent; or (iii) began using cannabis during mid-adolescence and persisted well into adulthood. Implications are discussed with respect to the 2020 referendum.