The implications of recent findings on the link between cannabis and psychosis

Recent studies have increased long-standing concerns that cannabis use can trigger or exacerbate psychosis and schizophrenia. Widely publicized, these findings have led to calls for tougher laws and/or more aggressive enforcement of prohibitions against cannabis use and distribution [1]. The findings may be confounded by unobserved factors. Even if they prove broadly correct, these findings would still have more implications for clinical practice than for broader public policy. Their policy marginality reflects the fact that the psychosis effects add modestly to the total damage from cannabis use. In addition, there is little evidence that tougher enforcement or laws effectively reduce use. However, these findings may be important for design of specific services to individuals with marijuana use disorders and to individuals at risk for psychiatric disorders Some studies report dose–response links between cannabis use and subsequent onset of schizophrenia [2,3]. A recent review article summarizes five strong studies, all of which report adjusted odds ratios exceeding 2.0 in assessing the link between cannabis use and schizophrenia [4]. Several studies indicate that cannabis may hasten the onset of schizophrenia symptoms among individuals predisposed to such disorders [5]. Other studies suggest gene–environment interactions within a small minority of marijuana users [6]. Still other studies raise additional concerns about the phenomenon of cannabis psychosis [5]. A recent follow-up study in Denmark indicates that approximately half of individuals hospitalized for cannabis-induced psychotic symptoms will be diagnosed subsequently with schizophrenia spectrum disorders [7]. A Dutch study showed that, among individuals with no baseline psychiatric disorders, cannabis users were more likely than others to display psychotic symptoms and disorders [8]. Among those with psychotic symptoms at baseline, cannabis use was associated with markedly worse outcomes. The most widely cited study is a cohort study of 1969–70 Swedish conscripts (97% of all Swedish males aged 18–20), followed over 25 years [3]. Although the authors control for several pertinent factors they cannot control for several important potential confounders, such as family genetic histories. The majority of excess schizophrenia risk was concentrated in the subgroup who had used cannabis more than 50 times during their life-time, with much more modest effects found among lighter users. POLICY IMPLICATIONS