Marijuana, Psychosis, and Policy Development

"Two things are needed to move beyond policy-biased appraisals of the evidence on cannabis and psychosis.

"First, we need to use explicit criteria to assess the evidence for contributory causal relationships and apply them in an even-handed and consistent way. We should avoid the example of the tobacco industry in setting such a high standard of evidence for a causal inference that no evidence can satisfy it (30). We should also avoid accepting weaker evidence in support of causal explanations, for example accepting observational evidence that persons with psychosis who use cannabis have better social adjustment than those who do not as evidence of the cognitive benefits of cannabis use [e.g., (31)].

"Second, we need more nuanced analyses of the relationships between evidence and policy than those often implicitly assumed [e.g., (32, 33)]. For example, accepting that regular cannabis use may play a contributory causal role in psychosis does not entail support for cannabis prohibition. There is experimental evidence, for example, that heavy alcohol use is a contributory cause of the psychosis delirium tremens (34). There is also observational evidence that sustained heavy alcohol use can produce psychoses that persist beyond alcohol withdrawal (35, 36). This evidence does not justify alcohol prohibition because policy makers have to consider the social and economic consequences of the policy, as revealed during national alcohol prohibition in the USA from 1920 to 1933 (37).

"Ideally democratic pluralist societies should decide on an appropriate cannabis policy by weighing the costs and benefits of cannabis use and cannabis control policies (38, 39). Policy makers need to weigh the harms that may arise from cannabis prohibition, such as, criminal records for cannabis users, production of a large illicit market, police corruption and discriminatory enforcement of the criminal law (38). The costs of cannabis prohibition and the potential benefits of regulating and taxing cannabis have led a majority of US citizens to support the legalization of adult cannabis use (40).

"If a government decides to legalize cannabis, however, the evidence on cannabis and psychosis is relevant in making decisions as to how cannabis should be regulated. Experience with alcohol (41), for example, suggests that we should discourage the use of high potency cannabis by basing taxes on the THC content of cannabis products or setting a cap on their THC content (42). The availability of cannabis retail outlets could also be limited and restrictions on the legal age of purchase enforced to reduce adolescent access (41, 43)."