Debate tends not to take into account the effects of cannabis on what is an at-risk section of the population, namely teenagers.

Social tolerance of cannabis use is higher than ever, but it is no time for complacency. A recent conference in Dublin organised by The Addiction Group at the Department of Public Health and Primary Health Care, TCD, highlighted the effects of cannabis use on the teenage brain, and gave insight into the current wave of research aimed at predicting use in young people.

In the last couple of years, the debate around cannabis use has changed from the old quarrel over what 'class' of drug it is legally, moving towards the possible medical benefits of cannabis - more specifically the active chemical cannabidiol - on the basis that it may have a positive role to play in the treatment of physical and mental ailments.

We have heard wonder-stories around the use of cannabidiol (otherwise known as CBD), for chronic pain, epilepsy, anxiety, depression, insomnia and more. There is clearly work to be done in establishing exactly what CBD has to offer. But there is also the risk that enthusiasm for the possible benefits will outweigh caution around the known harms.

Right now, cannabis is illegal in Ireland for recreational purposes, and use for medical purposes is approved on a case-by-case basis. However, there is, broadly speaking, much greater tolerance for cannabis use than there used to be, and there are valid reasons for that. But the debate tends not to take into account the effects of cannabis on what is an at-risk section of the population, namely teenagers.

There is sufficient evidence to show that cannabis has a greater effect on the brains of teenagers than adult users. As Professor Hugh Garavan, University of Vermont and formally Associate Professor of Psychology, Trinity College Institute of Neuroscience, says, "there's a good body of animal research showing this. The teenage brain is more plastic (ie, can change more) which is why teens can learn skills so quickly. But the downside of being so plastic is that their brains may be more susceptible to detrimental influences. The brain` has cannabinoid receptors (where cannabis binds/has its effects in the brain) and the numbers of these receptors increases a lot during puberty so the teenage brain may be especially sensitive to cannabis's effects."

As for whether those effects are necessarily harmful - that is more difficult to prove. "Demonstrating causation is really hard," Prof Garavan explains. "Most research compares users to non-users, but this type of research (known as cross-sectional research) is always going to be ambiguous. Any observed difference (eg, a cognitive impairment in the users) could have arisen from the use, or could have preceded the cannabis use (some pre-existing risk factor for use such as poor impulse control) or could just be correlated and unrelated (eg, maybe wealthier kids have the cash to buy cannabis so a whole bunch of factors related to being wealthier could be correlated with use but would not have caused or arisen from the use)."

However, without being alarmist, he says, "there is evidence of cognitive impairments in heavy users compared to non-users. There's evidence that these effects may reduce with abstinence which suggests that current chronic use may affect cognition."

And although he says, "we have to be cautious not to stigmatise users and sound alarms carelessly - there are lots of adult users who are high functioning," he also believes that the assumption that cannabis does have detrimental effects in teens is "a reasonable one, I think."

For Dr Gerry McCarney, Consultant Child and Adolescent Psychiatrist at the HSE National Drug Treatment Centre, the need for caution is very evident in his daily working life. "In the debate around whether to make cannabis more freely available, the serious issue of the harm it causes to the developing brain gets lost." Cannabis, he says, "is the primary drug of abuse in four out of five people who present to our service. Once you look at those who use two drugs or more, 90pc of those who seek our help are using cannabis. It is being used more than any other drug, and if we were to increase access, this will go up."

Ireland, he points out, isn't among the highest users of countries internationally, but, our use had started to come down five or six years ago, and has now begun to creep up again. In addition to that, there is a potency issue. Where the ratio between THC (the psychoactive ingredient) and cannabidiol has changed dramatically, from around 15:1, to around 80:1.

This increased potency, Gerry says, "has a serious impact - including paranoia, anxiety, and suicideality among those who use more and more regularly." Among those teenagers who smoke weed, even in small quantities, there is an elevated risk of chronic bronchitis, and what psychologists call 'diminution of lifetime achievement,' basically meaning a dearth of motivation and ambition; "career aspirations fall and school drop-out increases." Along with that are implications for short-term memory impairment, impaired motor coordination, altered judgement and risk-taking.

And, cannabis use can leave an impact on young people's capacity to perform, even when they stop using.

Much of the research currently being done into the topic is around the predictability of cannabis use among teenagers - identifying those who are likely to use in a way that will have an impact on their health - and here, the idea is to one day be able to give specific information around individual risk. As Prof Garavan says, "these days more and more people think that cannabis is fairly low risk, which may lead to more teens using it.

"However, I think teens can make rational decisions with regard to their drug use so demonstrating negative effects (ie proper scientific evidence rather than just preaching at kids) could help influence their decisions.

"Ultimately, it would be great to be able to give kids more personalised guidance - for example, to be able to tell a kid that given their family history of schizophrenia, their genetics, their personalities, their brain characteristics and so on that cannabis is likely a bad risky decision for them - that is, not a blanket prohibition ("all drugs are always bad for everyone all the time" tends to get ignored) but individualised, rational guidance could prove more effective in steering vulnerable kids away from certain drugs.





The big question: can we predict use?

⬤ So, the all-important question: is it possible to predict among young teenagers who will go on to use cannabis?

⬤ Yes, says Dr Garavan. "We have a study in which we followed teens for a number of years and were able to predict (not perfectly but with pretty high accuracy) which kids would start using. A mix of personality factors, early alcohol and cigarette use, life stressors and some brain measures could predict who would use."

⬤ Which is more accurate in terms of prediction, brain-changes or life-events? "Life events. What's interesting though is that the best predictors (life events and personality) tend to be more general in that they predict cannabis and binge drinking in boys and girls. The brain measures, in contrast, were much more specific in that the predictors of cannabis use in boys did not predict cannabis use in girls."

⬤ So there is a difference in male and female users, in terms of predictability? "Yes. That same analysis describe above was able to predict female users better than male users. This is notable as there were fewer female users and they smoked less, so it suggests that there is a more distinctive profile for female users."

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