Wayne Hall: Experimenting with marijuana

WHAT will happen if Californians vote on 2 November to legalise marijuana use by any adult over the age of 21? Let's ignore for the moment the vexed constitutional issues that will be raised if a US 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 the referendum's supporters, all Californians will be winners. The change will 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.

By contrast, opponents predict that the change will increase rates of marijuana use, and thus magnify the harm arising from that use. The consequences of greatest concern include more marijuana-related road traffic accidents and deaths; more psychoses and other 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 legalised cannabis use along these lines, has in fact tolerated only small-scale retail sales of marijuana in a limited number of coffee shops in its larger cities.

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.

Experience with alcohol suggests that marijuana will be available to underage users (as it is now), whatever the minimum legal age. Thus, legally available cheaper marijuana will probably mean more use by more young people, and probably more daily use later into adulthood. In general, the more people who use a drug, the larger the number of problem users there will be.

If current estimates are correct, about 10 per cent of users - and one in six who start in adolescence - may become dependent on marijuana.

Dependent users are more likely to experience road and workplace accidents, increased respiratory disease (if they smoke marijuana), exacerbations of some serious mental disorders, and impaired school and work performance.

It is uncertain how far these adverse effects can be mitigated by preventive measures such as implementing roadside drug-testing programmes, persuading users to use vaporisers rather than smoke joints, discouraging young people from using marijuana, and encouraging early treatment for problem users.Indeed, the scale of any increase in marijuana use will critically depend on how tightly regulated marijuana sales are - how many licensed growers there are, the number of sales outlets, their locations and trading hours, eligibility requirements for use, content of marijuana's psychoactive ingredient, and how much promotion is allowed.

It will be a major policy challenge to get this balance right.

Californians will have to decide how much weight to give to respect for adult liberty, protection of minors, avoidance of a large-scale black market, fiscal considerations, and protection of marijuana users' health.

If they vote for legalisation, 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 Australia fellow at the University of Queensland Centre for Clinical Research, Brisbane, Australia.