What will happen if Californians vote in November to legalize marijuana use by any adult over 21? Let's ignore for the moment the vexed constitutional issues that will be raised if a U.S. state enacts legislation that conflicts with federal law. Let's focus instead on what may happen if the law changes as the referendum proposes.
If we are to believe supporters, all Californians will be winners. The change will legitimize marijuana's de facto legal status since passage of Proposition 215 in 1996. It will thus reduce state expenditure on enforcing a widely violated law; remove marijuana growing and selling from the black market; enable any adult who wishes to use marijuana to do so; and introduce a tax on legal marijuana sales that will fill state coffers with revenue that formerly went to illegal growers (so long as there is no large-scale tax evasion).
By contrast, opponents say the change will increase rates of marijuana use, magnifying the harm arising from that use. The adverse consequences of greatest concern include more marijuana-related road traffic accidents and deaths; more psychoses and other serious mental health problems among heavy users; and heavier marijuana use by young people, negatively affecting their life chances. These effects, they argue, will more than offset any gains from tax revenue and savings from law enforcement.
It is difficult to know who is right, because no other country has adopted this policy. The Netherlands, which is popularly believed to have legalized cannabis use along these lines, has in fact tolerated only small-scale retail sales of marijuana in a limited number of coffee shops. Cultivation remains illegal and coffee shop sales are regulated.
Nevertheless, some broad predictions can be made. First, openly tolerating a market in a previously illegal commodity will expand the number of users, especially if the retail price is lower than the black-market price (as is proposed in the California bill). Experience with alcohol suggests that marijuana will be available to underage users, whatever the minimum legal age. This probably means more use by more young people, and probably more daily use later into adulthood.
If current estimates are correct, around 10 per cent of users may become dependent on marijuana. Only a minority of these problem users will seek professional help to quit. Dependent marijuana users are more likely to experience road and workplace accidents, impaired performance, increased respiratory disease and exacerbations of some serious mental disorders.
It is uncertain how far these effects can be mitigated by preventive measures, such as implementing roadside drug-testing programs, persuading users to use vaporizers rather than joints, discouraging young people from using, and enacting early treatment for problem users. Experience with alcohol suggests that these approaches may modestly reduce but not eliminate the harms, especially if sellers are allowed to promote their wares.
Indeed, the scale of any increase in marijuana use will critically depend on how tightly regulated sales are - how many licensed growers there are, the number of sales outlets, their locations and trading hours, eligibility requirements for use, content of THC, and how much promotion and advertising is allowed. If marijuana were allowed to be marketed like alcohol is now, expect more problem users than if it were regulated more like pharmaceutical drugs.
It will be a major policy challenge to get this balance right. Greater restrictions would provide incentives for illegal operators to remain, while less restrictive regulation would discourage black marketeers, but at the price of increasing the number of users and problem users.
Marijuana policy involves a choice between the evils of current repressive policies and the evils of increased use under a legal market. If Californians vote for legalization, it will probably be 20 years or more before anyone can say whether, on balance, they decided wisely.
Wayne Hall is a National Health and Medical Research Council fellow at the University of Queensland's Centre for Clinical Research.