How ideology shapes the evidence – a discussion paper.

An interesting paper this week  from John Macleod & Matthew Hickman of the Department of Social Medicine at the University of Bristol. Headed “for debate” the paper was called “How ideology shapes the evidence and the policy: what do we know about cannabis use and what should we do?” Read it in full here on Wiley Interscience.

The abstract states

In the United Kingdom, as in many places, cannabis use is considered substantially within a criminal justice rather than a public health paradigm with prevention policy embodied in the Misuse of Drugs Act. In 2002 the maximum custodial sentence tariff for cannabis possession under the Act was reduced from 5 to 2 years. Vigorous and vociferous public debate followed this decision, centred principally on the question of whether cannabis use caused schizophrenia. It was suggested that new and compelling evidence supporting this hypothesis had emerged since the re-classification decision was made, meaning that the decision should be reconsidered. The re-classification decision was reversed in 2008. We consider whether the strength of evidence on the psychological harms of cannabis has changed substantially and discuss the factors that may have influenced recent public discourse and policy decisions. We also consider evidence for other harms of cannabis use and public health implications of preventing cannabis use.We conclude that the strongest evidence of a possible causal relation between cannabis use and schizophrenia emerged more than 20 years ago and that the strength of more recent evidence may have been overstated—for a number of possible reasons. We also conclude that cannabis use is almost certainly harmful, mainly because of its intimate relation to tobacco use. The most rational policy on cannabis from a public health perspective would seem to be one able to achieve the benefit of reduced use in the population while minimizing social and other costs of the policy itself. Prohibition, whatever the sentence tariff associated with it, seems unlikely to fulfil these criteria.

Which is actually more or less what this site has been trying to say for some time now. Indeed, most of the paper is to be applauded although it falls short in some important ways. It starts by pointing out just how widespread cannabis use actually is

Around half of adults in the United Kingdom (including the last Home Secretary) report that they have tried cannabis, with approximately a fifth of young adults reporting regular use

It is this degree of widespread acceptance of cannabis use that really sets it apart from other illicit drug use, although far more people drink alcohol the level of cannabis use is non the less a significant fact of life, it is, to a large degree, normalised at least in some parts of society. It was this fact as much as anything that lead to the downgrading of cannabis to class C of the misuse of drugs act. The paper then goes on to describe the backlash against this move from some sections based on the claims of a link between cannabis use and mental illness

Indeed, to question the strength of the evidence that cannabis causes schizophrenia seems equivalent to some to encouraging the use of cannabis by young people (e.g. Melanie Phillips, ‘The Cabinet’s cannabis cabaret’, Daily Mail, 23 July 2007, ).

And then observes that

Against this background of apparent widespread opposition to a ‘softer’ stance on cannabis use, the decision to re-classify cannabis was reversed in 2008

The paper then goes on to examine the actual evidence that cannabis can cause schizophrenia and concludes

We continue to take the view that evidence that cannabis use causes schizophrenia is neither very new nor, by normal criteria, particularly compelling.

Next the paper addresses what it considers to be more serious health risks connected with cannabis – those around the issue of smoking. Here it  makes several strange and frankly innacurate claims. Firstly concerning smoking of pure cannabis:

Aside from the question of health risks of cannabis smoke alone [20,21],

Whilst of course we accept that any form of smoking carries health risks it’s interesting the authors quote the references they do in support of their claim that cannabis smoke alone carries serious health risks because that is specifically not the conclusions these studies come to. The references given [20,21] are familiar ones:

[20].  Hashibe M., Morgenstern H., Cui Y., Tashkin D. P., Zhang Z. F., Cozen W. et al. “Marijuana use and the risk of lung and upper aerodigestive tract cancers: results of a population based case–control study”. – read it here. This study concludes that

Our results may have been affected by selection bias or error in measuring lifetime exposure and confounder histories; but they suggest that the association of these cancers with marijuana, even long-term or heavy use, is not strong and may be below practically detectable limits.

[21]. Tashkin D. P. “Does smoking marijuana increase the risk of chronic obstructive pulmonary disease?” – read it here. This study concludes:

Given the consistently reported absence of an association between use of marijuana and abnormal diffusing capacity or signs of macroscopic emphysema, we can be close to concluding that smoking marijuana by itself does not lead to COPD.

The use of these studies to try to prove the opposite point does make me wonder if they actually read the papers quoted, or just assumed they would support the claims they were making. The paper then states:

the relatively poor combustion properties of cannabis mean that to facilitate use through smoking (the most common means of ingestion), most people mix cannabis with tobacco.

This just so wrong, statements like this only serve to demonstrate the authors have no first hand experience of the subject at hand and shows the danger of gaining knowledge from books as opposed to real life experience. Cannabis does not have relatively poor combustion properties if it’s prepared correctly and that is not the reason it is often smoked mixed with tobacco, indeed in many parts of the world cannabis users would never consider mixing the two drugs. However, they are correct to state that cannabis is most often smoked this way in our culture but the reasons for this are historical and cultural and as such are open to change.

This exposure to generally unfiltered tobacco smoke seems a clear and relatively unambiguous harm of cannabis use, albeit one that is difficult to quantify precisely.

Now we agree that breathing in the unfiltered tobacco smoke is very harmful, but not that it is a “relatively unambiguous harm of cannabis use” as such. Whereas UKCIA would agree that tobacco use is the most serious health issue surrounding cannabis use it is not an inevitable consequence of that use, it is something which could be changed and which has simply never been addressed by any aspect of policy.

The study points out that some users develop a dependence on cannabis and that some seek help, this is almost certainly true but of questionable severity. However, it then asks

Why so many commentators have chosen to focus not on these harms, but on the issue of cannabis and schizophrenia, is an interesting question in itself.

Aside from the fact that the issues identified are not in fact as serious or clear cut as they imply, the question is still an interesting one. To be honest the simple answer is probably because the smoking issue is so uncomfortable for politicians and the dependence issue so weak (at least for adults) while the “cannabis makes you mad” claim was so press-friendly. Insanity of course is something which scares people and makes lurid headlines. The paper discusses the issue without actually making that fairly obvious conclusion.

A further consideration that may be relevant is that in the ongoing debate around drug control, anti-prohibition arguments have often been framed in terms of overstatement of the strength of evidence for the harmless nature of cannabis use. Possibly this has led those ideologically opposed to the anti-prohibitionist position to overstate evidence for cannabis harm.

As this blog has observed several times, the “harmless herb” argument gave the prohibition campaigners the golden opportunity to argue against law reform by enabling them to paint campaigners as simply uncaring. The claims of a link to mental illness were ideal for their case, affecting as it does young people in such an emotive way.

The paper points out that cannabis is associated with poor outcomes for ill people but outlines the weak case which has been made for a causal link. Indeed it says of the claims of a genetic explanation for a link between cannabis and mental illness

Several leading geneticists and epidemiologists have argued that, rather than evidencing any substantive causal effects, these studies demonstrate many of the pitfalls around studying gene–environment interactions. Concern has also been voiced that premature acceptance of the face validity of some of this evidence may have led to inappropriate and unhealthy changes in research funding policy

In a final swipe against the mental health claims the paper looks at the likely effect of trying to reduce the rates of mental illness by preventing cannabis use

We can also, within the same assumptions around causality, estimate the number of people whose cannabis use we would need to modify to a specified extent to prevent one additional case of schizophrenia. For example, our recent modelling suggests that we would need to prevent between 3000 and 5000 cases of heavy cannabis use among young men and women to prevent one case of schizophrenia and that four or five times more young people would need to avoid light cannabis use to prevent a single schizophrenia case

To be clear, what we are suggesting is not that prevention of cannabis use is pointless. Rather, based on available evidence, even if cannabis use does cause schizophrenia, prevention of schizophrenia is unlikely to be the main objective motivating our cannabis prevention efforts.

But the prevention of serious mental illness was precisely the reason given for the recent upgrading and subsequently more repressive regime toward cannabis. This study is saying quite clearly that the logic behind the upgrade was flawed. However, the paper is also claiming that prevention of cannabis is desirable and it goes on to explain why:

In our view, the main reason to prevent cannabis use is to prevent young people smoking the tobacco they smoke typically with cannabis

Now that is frankly an unbelievable comment. How can they seriously propose that a an argument for preventing cannabis use is that it is connected with the smoking of tobacco when no attempt has ever been made to break that association?  Not to mention of course that their logic in arriving at this linkage between tobacco and cannabis is itself flawed.

This leaves the concern over dependence, but of course that is a problem prohibition has shown itself to be pretty useless at addressing.

The authors also state that a goal of policy should be

and as long as cannabis use continues to be prohibited, preventing the criminalization of young people is also desirable.

Which would seem to rule out criminal penalties for cannabis use. The paper correctly observes that

The only important possible benefit of prohibition is prevention of cannabis use. There is little or no evidence that it effectively achieves this benefit.

So it would seem reasonable to conclude the authors support legalisation of cannabis use, although they don’t quite manage to say so. Instead they write

In our view, cannabis use should be reframed as a public health rather than criminal justice issue, and the important public health goal of prevention should be pursued within the same paradigm as is applied currently to the two substances that cause the most public health harm: tobacco and alcohol.

In other words, legalised and properly controlled and regulated, so why don’t they say it?

All in all this paper is an interesting contribution to the debate about cannabis, although spoiled by a demonstrable lack of real world experience and an over willingness to use evidence that doesn’t exist concerning the harms related to smoking pure cannabis to justify the case for prevention.  It is a pity they messed up the smoking issue so badly and didn’t provide the argument for a properly run safer smoking campaign along the lines of UKCIA’s “Toke pure“. I did contact the authors at the address given to raise these points and was hoping to include their reply in this blog, but so far no reply has been forthcoming. You can contact them by mailing this address