The question of whether cannabis represents a gateway to harder drugs has occupied clinicians and researchers for 30 years. The reason for the interest in this question has been that a transition to harder drugs – heroin, amphetamines or cocaine – represents a significant increase in the degree of risk to which the individual is exposed. Even though cannabis is more psychotoxic than heroin, intravenous heroin abuse is in many other respects a considerably more serious condition than cannabis dependence. With heroin – which is also much more expensive than cannabis –, addiction not infrequently develops quickly, the dependence is strong, and mortality is considerably higher, mainly owing to overdoses. It is the rule rather than the exception that heroin-dependent individuals become socially marginalised. The abuse of both amphetamines and cocaine (not least in the form of “crack”) also leads to rapid development of severe dependence, with the risk of a series of mental side-effects and high mortality levels. Moreover, transition to intravenous abuse adds the risk of HIV infection.
The most extensive studies in this field have been carried out by Denise Kandel and her research team. They showed, at an early stage, how young people in the United States tended to progress through a sequence of increasingly strong drugs. A very large proportion of their subjects followed a series of graded steps, and very few deviated from the progressive sequence or hierarchy expected. Four stages were identified: 1) beer and wine; 2) cigarettes and spirits; 3) marijuana; and 4) other illegal drugs (). It was also found that the younger the age at which abuse started, the higher in the drug hierarchy the individual would climb; and the more intensive the abuse at any given stage, the greater was the risk that the individual concerned would progress to the next stage. Research carried out in the Nordic countries has found similar tendencies (). Golub and Johnson () have shown how the importance of alcohol as a gateway to abuse of hard drugs has declined while that of marijuana has increased. This is considered to be due primarily to the strong expansion of marijuana smoking observed in the United States over the past three decades. Yamaguchi and Kandel () have also shown how extremely rare it is for abusers of hard drugs not to have progressed via marijuana. Here, though, it should be emphasised that it is only a small fraction of those who smoke cannabis that ever try other illegal drugs, and even fewer who continue using such other drugs.
In general, then, prior cannabis use seems to be a necessary condition for the use of hard drugs; but is cannabis also a gateway to harder drugs in the sense that cannabis abuse is actually the cause of heroin, cocaine or amphetamine abuse? A great deal of effort has been devoted to answering this question. Since the vast majority of cannabis smokers do not become abusers of hard drugs, there is clearly no simple causal connection. What we can conclude from the various studies, in other words, is that cannabis is only one of the factors that seem to predispose an individual to the abuse of hard drugs. A range of other negative social and psychological background factors are also very important.
Kandel et al. () demonstrated a direct association between the intensity of cannabis abuse and the risk of progression to hard drugs. Of the subjects in their study group who had used marijuana more than 1,000 times in the course of their lives, 90 per cent had also tried hard drugs; of those who had used marijuana between 100 and 1,000 times, 79 per cent had used hard drugs; and of those who had used marijuana fewer than 100 times but more than 10 times, 51 per cent had used hard drugs. Of those who had used marijuana between 1 and 9 times, 16 per cent had also used hard drugs. Among the subjects who had never used marijuana, though, only 6 per cent had used hard drugs.
This statistical association between the intensity of cannabis consumption and the likelihood of using hard drugs strengthens the case for assuming that there is a causal connection between cannabis smoking and progression to harder drugs, but it does not constitute proof of such a causal connection.
The association can also be demonstrated by means of the below findings from a more recent analysis of the study group from Newcomb’s () large-scale longitudinal study (of which an account will be given in the next section). However, this analysis concerns the proportion of the marijuana-dependent individuals in the group who also became cocaine-dependent – in other words, the proportion of the large-scale consumers/abusers of marijuana who also became large-scale consumers/abusers of cocaine. In all, marijuana-dependent subjects accounted for 8.1 per cent of the group of 27–29-year-olds studied. Of these, 36 per cent were dependent on cocaine as well. Kleber () has pointed out that studies linked to the Center on Addiction and Substance Abuse at Columbia University have shown that 60 per cent of the young Americans who use marijuana before the age of 15 will use cocaine later in life. Further, it has been shown that young American between the ages of 12 and 17 who use cannabis are 85 times more likely to use cocaine than non-smokers of cannabis in the same age group.
The general impression, then, has been that the imperative role of cannabis in the “stepping-stone” model has resisted all attempts to prove it scientifically. On the other hand, a large body of circumstantial evidence has been gathered. It is found time and again that cannabis is a central component of the network of influencing factors that leads to the abuse of hard drugs.
This discussion has continued with unabated intensity. Some people claim that this is the crucial question of the debate on the dangerousness of cannabis: “If the gateway theory is incorrect, cannabis cannot really be all that dangerous, can it?” The intensity of the defence of the gateway theory has often been based on the assumption that the correctness or otherwise of this theory determines what the correct view on cannabis should be. Since I account for many other suspected and proven harmful effects of cannabis abuse in this report, I obviously do not share that assumption.
Is the Question of the Role of Cannabis As a Gateway to Hard Drugs Now Being Answered?
In a few recent research reports, Fergusson et al. () account for a number of analyses from a study of young people where attempts are made to find links between cannabis smoking and a range of developmental problems that might conceivably be affected by cannabis abuse. The study monitors 1,265 New Zealand children from birth; the subjects were 20–21 years old at the time of the latest report. The researchers have unique knowledge about the children and their backgrounds, and their analyses are greatly facilitated and given considerably larger weight by the prospective and longitudinal design of the study.
We will here look at only one of the issues addressed by the project: the “gateway” question. At the time of the first analysis, the subjects were 18 years old. Like many researchers before them – not least Kandel’s team – Fergusson et al. found that the initial analysis showed associations which were not, when other known factors were controlled for, very strong at all. The researchers then joined above all Kandel, but many other researchers as well, in suggesting an alternative explanation.
As the young people came into contact with cannabis, the researchers claimed, the effects caused by cannabis on the mind, the culture surrounding the drug, new friends who had acquaintances in abuser circles as well as other circumstances all combined to amplify the impact of negative background conditions. This hypothetical process is called a “cascade effect” (). In this way, then, cannabis would indirectly have caused or contributed to a number of difficulties encountered by the young people, including progression to other illegal drugs. This was obviously bad enough in its own right, but there had been no convincing proof that cannabis constituted a specific causal factor.
At the time of the later analyses, the subjects were 21 years old. The researchers then had more data at their disposal and used more advanced methods of analysis. It was possible not only to take more careful account of other factors known to contribute to negative developments, but also to control, to a degree, for time-dependent and not fully known influencing factors. The more intensive the cannabis abuse, the stronger was the association with use of other drugs. For younger (14–15 years old) large-scale consumers in particular, there was found to be a very strong association even after controlling for other known or suspected causal factors ().
Here, then, a different picture emerges. Cannabis abuse is here an independent specific factor, in all likelihood a cause of the progression to “hard drugs”. This is the first time that associations of this strength have been found, and the theory of cannabis as a gateway seems to be on its way to being proved. If this study is replicated by another research team, it will be hard to dismiss the conclusions that could be drawn from the findings. Still, though, while this makes it clearer that there is a causal connection, we do not yet know the nature of that causal connection.
Since the existence of a strong association between cannabis use and a negative effect tends to make one think in terms of some kind of neurophysiological process rather than the psychosocial mechanisms which are generally close at hand, I will mention an interesting line of research and its accompanying hypotheses. This relates to the phenomenon of sensitisation, a sort of “inverse tolerance effect” where an addictive substance increases a person’s sensitivity to the euphorising effects of that substance. Even though most of the research is based on animal experiments, similar phenomena exist in humans. Not least interesting is the occurrence of cross-sensitisation. This means that exposure to one preparation (e.g. cannabis) should be able to make a person more sensitive to another preparation (e.g. heroin). But let me repeat once again that this is hypothetical reasoning ().
Selections from the book: “Adverse Health Consequences of Cannabis Use. A Survey of Scientific Studies Published up to and including the Autumn of 2003”