Commentary on Hickman <i>et al.</i> (2009): The place of risk in drug policies

The calculations by Hickman and colleagues [1] are necessarily approximate. They rely on general population surveys concerning incidence of psychiatric conditions and on cohort studies, through meta-analyses, for the relative risks based on the association of cannabis use with psychosis. This is an association where, as they and major reviews [e.g. 2, 3] acknowledge, the causal connection remains in contention. But the calculations are nevertheless very useful, giving us a sense of the order of magnitude of the potential risks and allowing us to put them in a comparative perspective with other risks from substance use and common behaviours. Are the risks as Hickman et al. calculate them high or low? There is of course no absolute answer to the question, but we can put them into perspective by comparing them with other risks of daily life. Hickman and colleagues do, themselves, provide a comparative frame in terms of ‘number needed to treat’ (NNT) to prevent cases of various diseases. By this comparison, the NNT in preventing cannabis use to prevent one case of schizophrenia is more than an order of magnitude higher than the NNTs for the three preventive scenarios they cite for other diseases. But in considering drug policy, particularly a prohibitive policy as in the UK for cannabis, it is arguably more relevant to consider the relative risk of the behaviour itself. One standard comparison here is in terms of lifetime risk of death from a behaviour. The outcome in terms of which Hickman et al. are calculating is the onset of a mental illness, which is less final and usually considered a lesser adverse fate than death. Recasting their calculation in terms of the risk of death would probably make the odds at least a further order of magnitude smaller. But if we accept their frame and make calculations based on the numbers they give for absolute risk of psychotic and schizophrenic disorders, and on the relative risks from the meta-analyses, then the lifetime risk for developing schizophrenic disorders given adult lifetime heavy cannabis use would be about 1.1% for men and 0.4% for women, and given adult lifetime lighter cannabis use would be 0.7% for men, and 0.3% for women. These calculations assume that the cannabis use is ongoing at the level described from age 16 to age 39 inclusive without any interruption, obviously a very unrealistic assumption. For psychotic disorders, the respective lifetime risks were 2.5% (M) and 1.6% (W) for heavy cannabis consumption, and 1.6% (M) and 1.0% (W) for lighter use. Up-to-date calculations for lifetime risks of death are surprisingly scarce. But if we take a compilation in the US in 1979 [4] as a basis, each of the following weekly behaviours if kept up for 50 years would carry a risk of death of 1 in 100: travelling 6189 km by jet plane every week; 1857 km by car; 62 km by bicycle; one hour by canoe; or smoking 5 cigarettes per week. Recent calculations underlying the new Australian guidelines on low-risk alcohol consumption [5, 6] imply that the risk of dying of an alcohol-related cause for a man drinking up to the current UK ‘sensible drinking’ guideline [7] for males of no more than four UK standard drinks per day is about 3.1 in 100, and for a woman drinking up to the guideline for females of three UK standard drinks per day about 1.4 in 100 [6, pp. 48–49]. By these standards, the risks that Hickman et al. calculate are well within the range of what is commonly found to be acceptable risks for voluntary risky behaviours. However, it should be noted that possible adverse effects of cannabis are not limited to schizophrenia and psychosis. Risks are better established, in fact, for other adverse outcomes of cannabis use, such as driving casualties [8], although the total risks seem to be substantially less than for alcohol, for instance [9, 10]. So why the current focus on schizophrenia, which seems to have been a key consideration, as Hickman and colleagues note, in the British government's decision to reclassify cannabis back to a more ‘dangerous’ category? One possible answer is in the quandaries of a particular profession: psychiatry. When it comes to behavioural preventive approaches, psychiatry has had relatively little to offer, at least in its heartland of serious mental illness such as schizophrenia or psychosis. Preventing schizophrenia by tackling cannabis use may have thus seemed an opportunity for psychiatrists to make the most of. A second answer, clearly, is that values trump risk calculations. When there is a pre-existing normative position, calculating actual levels of risk is brushed aside and seen as irrelevant or even an impediment. Any level of risk, however small, becomes an apparently scientific justification for the desired policy outcome. But risk is a part of the human condition, and a rational drug policy cannot be based on its complete elimination. Calculations like those of Hickman et al. point the way to a more reasoned discussion of drug policies. None

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