Regular Adolescent Cannabis Users More Likely To Have Later Problems with Drugs, Alcohol and Tobacco Interview with:
Dr Michelle Taylor PhD
Senior Research Associate in Epidemiology
MRC Integrative Epidemiology Unit (IEU)
School of Social and Community Medicine
University of Bristol
Bristol UK What is the background for this study?

Response: Many previous studies have looked at adolescent cannabis use, however most of these look at use at a single time point, for example whether an individual has ever used cannabis at age 16 years, or how regularly a person uses cannabis at age 18. However, as young people do not initiate use at the same time or follow the same pattern of use, using measures at a single time point does not always tell the whole story.
We used a form of statistical modelling using data taken over the course of adolescence to try and characterise underlying patterns of cannabis use across adolescence. We used data from the Avon Longitudinal Study of Parents and Children which had information on cannabis use at six time points between the ages of 13 and 18 years. What are the main findings?

Response: We found that the prevalence of occasional and regular cannabis increased between the ages of 13 and 18 years. We identified four patterns of cannabis use that people might follow. 80% of our sample were classified as non-users of cannabis, 14% were what we called late-onset occasional users, meaning that they generally started using cannabis later in adolescence and only used it occasionally. There were also a group of early-onset occasional users and a set of regular users who started using cannabis early in adolescence and regularly used the substance across the time period.

We then looked at the association between adolescent cannabis use patterns and substance use at age 21 years. We found that regular adolescent cannabis users were 37 times more likely to be tobacco dependent. They were also 26 times more likely to have used other illicit drugs and 3 times more likely to have harmful levels of alcohol consumption. What should clinicians and patients take away from your report?

Response: Our research supports previous evidence that cannabis is a ‘gateway’ to the use of other substances. However, it is important to note the limitations.

The main one here is that we have provided evidence of correlation and not causation. There are many possible explanations for the observed correlation that we see between different drug classes. One is the biological explanation that cannabis alters the way your brain works and therefore primes you for wanted or needing the next substance. There are also many environmental explanations, an example of this being drug availability. Cannabis could be earlier in the sequence of drug use because it is easier to get hold of and less of a taboo than other drugs.

Despite the fact that we cannot yet say whether there is a causal link, research of this kind does lend itself to public health and the development of interventions as we can still identify the subgroups that are likely to be at risk of problematic substance use. What recommendations do you have for future research as a result of this study?

Response: I think that future research needs to focus on working out whether there is a causal link between cannabis use and other drug use. It is not possible to do a randomized controlled trial, which is the gold standard in causal inference, for ethnical reasons. However, recently there has been a lot of development of causal inference methods in epidemiological studies that could be applied. Is there anything else you would like to add?

Response: Cannabis is a drug that receives a lot of media attention and is subject to many debates surrounding its legality. I think it is important that even though we do not have solid proof that cannabis is harmful does not mean that is isn’t. I would urge anyone thinking of trying cannabis to be aware of the possible outcomes in making their decision.
Neither myself or my co-authors have any disclosures. Thank you for your contribution to the community.

Michelle Taylor, Simon M Collin, Marcus R Munafò, John MacLeod, Matthew Hickman, Jon Heron. Patterns of cannabis use during adolescence and their association with harmful substance use behaviour: findings from a UK birth cohort. Journal of Epidemiology and Community Health, 2017; jech-2016-208503 DOI: 10.1136/jech-2016-208503

Note: Content is Not intended as medical advice. Please consult your health care provider regarding your specific medical condition and questions.

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