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Major Studies of Drugs and Drug Policy
Canadian Senate Special Committee on Illegal Drugs
Volume I - General Orientation

Chapter 7

Cannabis: Effects and Consequences

Cannabis, as we saw in Chapter 5, acts on the central and peripheral nervous systems in various ways. While research has established a fairly clear role for some of the components of cannabis, D9THC in particular, the main active component, we are less sure of the role of other chemicals. Similarly research, which is often conducted on laboratory animals or in an even more specialized manner on molecules extracted for experimental purposes, does not reflect the conditions under which the average user uses marijuana. We have seen that, in some cases, dosages used for experimental purposes on laboratory animals, in particular to determine the chronic effects of regular use, are dosages that are unimaginable for humans, the equivalent of 570 marijuana cigarettes a day. Since THC content varies greatly with the cannabis available on the market, since users make different use of the drug depending on the place and circumstance, and since individual characteristics interact with the actual effects of cannabis, knowledge of the effects is necessarily influenced.

From an even more technical standpoint, we should point out that a statistical association – that is, the fact that two facts are concomitant – in no way indicates causality. To infer causality, a certain number of methodological prerequisites must be satisfied. In addition to the statistical association, we must be able to dismiss chance and alternative hypotheses, and show that the causative factor does precede the inferred consequence. According to the WHO:


Causal inferences can be drawn from research findings by judging the extent to which the evidence meets widely accepted criteria. These include: strength of association, consistency of association, specificity, dose-response, biological plausibility, and coherence with other knowledge. These criteria are not sufficient to show that an association is causal but the more are met, the more likely it is that the association is causal. [1][1]  


Moreover, a strong tradition in the philosophy of science holds that you can never prove a hypothesis. The most you can do is falsify – that is, dismiss – alternative hypotheses.[2][2]

To try to meet the requirements of causality, researchers have developed sophisticated research methods, providing in particular for the random selection of subjects for a study, the random assignment to experimental conditions and non‑experimental conditions (control group), the use of double blind and placebo techniques, the careful control of intervening variables that could represent as many alternative hypotheses as researchers are trying to eliminate. This is how, for example, they usually try to test medications that are put on the market.

For most questions involving human behaviour, a fortiori in society, it is difficult and rarely possible to establish such a causality relationship for the simple reason that each of these methodological requirements can rarely be met. In our case, the effects of cannabis use, the methodological constraints are particularly obvious. We cannot gather a random sample of cannabis users since we don’t know the population. Therefore we must rely on alternative methods for selecting subjects (volunteers, for example). It is difficult to have people smoke cannabis who would otherwise never use it38 without running the risk of contravening certain rules of ethics, or even legal provisions. And if we resort to people who have already smoked it, there is necessarily contamination of the control group. The cannabis that is used in the lab may be completely different from that of users who buy it off the street. And controlled laboratory conditions definitely do not reproduce the methods of cannabis use, which we know are usually a form of social ritual. As for studies–and they are numerous–conducted on animals (monkeys, mice, rats…), they may be interesting, but the possibility of transposing their results onto humans is limited. Lastly, we note that, as most cannabis smokers also smoke tobacco and drink alcohol, it is difficult to separate the effects of one from the effects of the others.

Obviously that does not prevent researchers from conducting studies, and these studies are also necessary. However, it does require researchers to be as prudent as possible when interpreting their results, in particular with respect to the ability to generalize about all marijuana users and to draw causal inferences. This is a caution that we do not always find, far from it, as this chapter will repeatedly show.

Lastly, we should note the distinction between effects and consequences. Smoking cannabis has immediate effects, some physiological and some psychosocial, that we must describe. But smoking cannabis, especially repeatedly, can also have consequences, some immediate – for example, the ability to perform certain tasks or the ability to drive a vehicle – and others more distant – for example if smoking cannabis results in a greater risk of lung cancer and if it has a lasting effect on memory.

We are aware of just how arbitrary these distinctions can be insofar as a human being is a whole, an organism integrated into his emotional and social environment and into his ecosystem. The physiological, psychological and social effects interact with one another, infiltrate one another, influence one another and act together rather than separately. In some ways, these distinctions remain the reflection of our incompetence, or at least of our inability, to think about the various systems of a human being as a whole, from every angle. This same incompetence can, also in part, explain the difficulty we have in creating a drug policy. It is to be hoped that those who come after us will be able to develop an integrated, holistic approach. For now, we are forced to use the means at our disposal, our fragmented understanding.

One last preliminary note. We were constantly guided by the need to be rigorous. Be that as it may, our resources did not enable us to be completely thorough and to examine the studies one by one for all these questions. In total, we know that approximately ten thousand studies have been published on cannabis over the last forty years! However, as Nelson points out, “Although the total volume of this literature is somewhat daunting at first glance, a sampling of the material soon reveals that much is repetitive and a relatively small number of papers are continually referred to by most authors.” [3][4] Despite this repetition, we could not go without examining a certain number of these studies. That is why we commissioned the preparation of a summary report[4][5] and also examined the summaries of scientific literature that were prepared in recent years.[5][6]

This chapter is divided into five sections. The first is a collection of statements on the presumed effects of marijuana that the Committee heard or that it was made aware of through its research. The following three sections examine the acute effects of cannabis, followed in turn by the physiological and neurological consequences, the psychological consequences and the social consequences. Then, because of its significance and the central place it holds in social and political concerns, we turn our attention specifically to the question of dependence possibly arising from prolonged use of cannabis.



[1][1] World Health Organization (1997), op. cit., page: 3; on this question also see: Hall, W. (1987) “A simplified logic of causal inference” Australian and New Zealand Journal of Psychiatry, 21: 507-513.

[2][2]  On this subject, see the works of Karl Popper in particular (1978 for the French edition) The Logic of Scientific Discovery Paris: Payot, and (1985) La connaissance objective. Bruxelles: Complexe.

[3][3]  It is even a little ironic that the National Institute on Drug Abuse (NIDA) in the US finances studies that have people smoke when the Institute believes that cannabis is a gateway drug: for example, see the study by Haney, M. et al. (1999) “Abstinence symptoms following smoked marijuana in humans” Psychopharmacology, 141; 395-404.

[3][4]  Nelson, P.L. (1993) “A critical review of the research literature concerning some biological and psychological effects of cannabis” in Advisory Committee on Illicit Drugs (eds.) Cannabis and the law in Queensland: A discussion paper. Brisbane: Criminal Justice Commission of Queensland.

[4][5] Wheelock, B. (2002) op. cit.

[5][6]  In particular the previously mentioned INSERM report (2001), op. cit. and the report from the International Scientific Conference on Cannabis (2002); as well as the report from the National Institute of Medicine in the US and the book edited by Professor Kalant, one of our witnesses.

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