• Cannabis;
  • evidence;
  • public health;
  • schizophrenia

We are glad that our invited commentary [1] on the relationship between cannabis evidence and cannabis policy has stimulated debate, and we are grateful to our scientific colleagues for their thoughtful responses [2–6]. These responses raise more issues than we can address adequately here, so we will stick to the main points. Our paper had two aims. The first was to examine, and attempt to understand, the recent scientific debate around possible cannabis harms. The second was to discuss what a policy around cannabis based on this evidence might look like. We recognize that political support for evidence-based policy in this context may be, at best, rhetorical.

Some of our commentators criticize our focus on evidence around cannabis and psychosis. This seems a little unfair because, for the past decade, and not just in the United Kingdom, this possible harm of cannabis use has driven the policy debate [7]. David Fergusson [2] was unhappy with our distinction between evidence on psychotic symptoms and evidence on schizophrenia. Again, this criticism seems misguided—we did not ignore the former, but the fact that we accorded a different status to the latter is simply normal epidemiological practice. In the same way, a cardiovascular epidemiologist would accord a different status to associations between stress and chest pain compared to objective evidence of coronary vascular disease [8,9]. Professor Fergusson [2] has also misunderstood our critique of the cannabis psychosis/schizophrenia evidence if he thinks it rests on ‘increasingly elaborate’ arguments. It rests now, and always has, on a very simple argument. Apparent independent effects of cannabis use on risk of psychosis may be due to residual confounding and measurement error [10]. That is not to say they are not causal—they might be, but it is simply impossible to know. Most scientists, including our commentators, agree on this. All we appear to be arguing about is the level of uncertainty.

What about other possible harms? Cannabis use has been associated with several adverse outcomes, as listed by Professor Wittchen [3], although as we have discussed elsewhere, in relation to most of these the strength of the evidence that the association has a causal basis is weaker than in the case of psychosis [11]. We agree with Hall & Degenhardt [4] that cannabis dependence can be added safely to the list of cannabis harms and apologize if we appeared to downplay the importance of this. Naively, we thought that another point we could all agree upon would be the harm that cannabis causes, through concomitant tobacco use. Can we be very clear that our assertions around tobacco and cannabis bear absolutely no relation to any pet thesis we are trying to promote and are not influenced by any ‘wish bias’? Perhaps we are hoist by our own petard here: guilty of over-interpreting the limited empirical evidence that a substantial proportion of cannabis users smoke cannabis mixed with tobacco, and that for many of them their cannabis use reinforces their tobacco use [12–14]. Obviously, if in most of the cannabis-smoking world, cannabis is not smoked with tobacco then our assertions in this regard are unlikely to be true.

However, even if we cannot agree on the precise hierarchical structure of a list of possible cannabis harms we seem to have come to a point where we can agree on the list and the fact that, based on the precautionary principle, we have a basis to advocate prevention. The question then is how do we pursue this goal? We believe that any policy be judged on the simple criteria used commonly to guide decisions around public health interventions: are they cost-effective, do they cause more good than harm and are they acceptable to people at whom they are aimed? Robert MacCoun [5] found our suggestion that cannabis prohibition probably failed this test ‘awfully brash’. This surprised us; all our respondents who commented on the issue agreed that there is no strong evidence that prohibition reduces cannabis use. Alongside this lack of evidence of benefit, prohibition also incurs considerable costs [15,16]. As MacCoun [15] suggests pessimistically, the alternatives could always be worse. This is true, and is the reason why the alternatives should be evaluated rigorously.


  1. Top of page
  2. Declarations of interest
  3. References