Does the use of cannabis predicate the use of other illegal drugs ? The study quoted below shows that the risk of someone using other illegal drugs is 90 times higher for 16 – 17 year olds who used cannabis at least weekly. It is essential that parents who believe their child is involved in cannabis use tackle this situation and do not turn a blind eye. Read the story (at the end of this article) by Ginger Katz….

There are two main difficulties to clarifying that marijuana is a gateway substance, but even so there is some good evidence.

The first difficulty is varying definitions of “gateway”. If one defines it as to how rare is the case of someone using other illicit drugs without ever using mj before that, there can be no dispute that a gateway effect or phenomenon exists. If one defines it as an absolute, as in, anyone who uses marijuana will use other illicit drugs, then it is clearly not true, since most who use marijuana don’t progress to other drugs. I think the meaning you’re using here, John, (correct me if not) is whether there are aspects to marijuana use that directly increase the odds that other drugs will be used, rather than just situational or reverse-correlation explanations for why other illicit drug use is so rarely found without prior marijuana use.

The second difficulty is that causality is complex and multi-faceted: even if direct causality is involved, some of the other trends that cause people to question causality (e.g. that early marijuana use is a sign of troubled development, which itself can account for increased likelihood of later use of other substances) are also true in many cases.

All the above not withstanding, one of the best studies I have seen to document the direct role of marijuana in later illicit drug use is one done in New Zealand, reported in the article “The developmental antecedents of illicit drug use: Evidence from a 25-year longitudinal study”, by David M. Fergusson, Joseph M. Boden, and L. John Horwood. The journal citation is Drug and Alcohol Dependence 96 (2008) 165-177. They looked at many potential risk factors and found predictive associations from childhood based on parental use, on the youth’s exposure to sexual abuse in childhood, on gender (male was at more risk), on novelty-seeking, and on childhood conduct disorders. They then moved into additional analysis where, “the statistical model was extended and refined by the inclusion of a series of time-dynamic covariates and controls for reverse causality.” Sifting through associations that included cannabis use, association with substance-using peers, alcohol use, cigarette smoking, and novelty-seeking, they found that except for some persistence in the novelty seeking factor, “accounting for substance use and peer factors reduced the associations
between the childhood fixed factors and illicit drug use and abuse/dependence to statistical non-significance.”

They then focus on cannabis use: “of the time-dynamic factors included in the final models, cannabis use had the largest and most complex associations. In particular, the study findings suggested an interactive relationship between age and the use of cannabis in the development of other forms of illicit drug involvement. In this relationship the effects of cannabis use were the strongest at younger ages, and declined progressively with age. Furthermore, the size of association depended on the extent of use of cannabis. The net results of these findings is that risks of illicit drug use were over 90 times higher amongst 16-17-year olds who used cannabis at least weekly when compared to non-users of cannabis. By the age of 25, these risks had reduced to nearly eight times higher. In addition, these associations were controlled for reverse causality by including a lagged measure of other illicit drug use in the model. These findings are consistent with the view that exposure to cannabis use increases risks of other forms of illicit drug use and abuse/dependence, even when allowance is taken of childhood factors and possible reverse causal associations.”

The authors note that the finding that “much of the association between childhood factors and other forms of illicit drug use and abuse/dependence was mediated via cannabis use” is important “in the light of claims that the association between cannabis and other forms of illicit drug use can be explained by common childhood factors …” “The present study suggests quite the opposite conclusion in which cannabis use mediated the effects of childhood factors on later illicit drug abuse.
Source: Alan Markwood of www.chestnut.org reporting to DrugWatch International. Oct.2009