Longitudinal cohort study finds adolescent cannabis users at greater risk of cognitive damage


in Cannabis

Cannabis could be harmful to cognitive development in under 18s

A collaborative team from Kings College University and Duke University have reported new findings suggesting that individuals who start using cannabis below the age of 18 risk permanent cognitive damage. The study was carried out as part of ongoing research on the 1000-strong birth cohort of Dunedin, New Zealand. Around 5% of this group used cannabis at least once a week in adolescence, and in subsequent cognitive testing over the years (the sample is now aged 38) this regular early user group scored lower on various cognitive tests, including having an average drop of  8 IQ points. This suggests that cannabis might be detrimental to brain development processes still ongoing in late adolescence – but importantly, those who started smoking cannabis after age 18 did not suffer any negative cognitive effects.

As a large longitudinal study, this provides some of the best data possible to those seeking a harm-minimisation drugs policy. One worrying consideration is the fact that the “street” cannabis available when this sample were teenagers (1985-90) would have had a lower delta-9-tetrahydracannabinol (THC) content than a comparable dosage today. Making the reasonable assumption that (as the primary psychoactive ingredient in cannabis) THC is a probable cause of these negative cognitive/developmental effect in adolescent consumers, we should perhaps even assume that this study underestimates the harm being done by cannabis consumption in under-18s.

Therefore, it follows, we should adopt the policy which will reduce adolescent consumption of THC by the largest margin. Here there is a strong case to be made that prohibition of cannabis is causing more harm than other options – cannabis is consumed by between 119 million and 224 million users aged 15-64 years worldwide, with around 6% and 8% of 16-24 year olds consuming the drug in the UK and USA respectively (UN Drug Report, 2010). Not only are teenagers generally more likely to take a prohibited substance (adolescence is almost tautologous with increased risk-taking behaviour), but furthermore, leaving the supply of cannabis to the “street” market has resulted in a doubling of average THC content in the last decade or so.  This is mirrored by a corresponding decrease in cannabidiol (CBD), another major cannabinoid which is thought to be neuroprotective and neutralise the effects of THC. There are also indications that countries where personal cannabis use is not prohibited (or at least not in practice), lower rates of consumption are typically seen, perhaps because of improved drug education and support services.

As a last point to consider, the fact that no difference was found in IQ or other cognitive measures between cannabis users and non-cannabis users who started over the age of 18 is also extremely interesting, and perhaps forms a contrast with similar research concerning alcohol consumption, which is worryingly inconsistent on this issue.

Taking a reasoned view then, the most that this research should be taken to suggest is just – exactly – that cannabis should not be consumed by those under the age of 18, a proposition wholly endorsed by the Beckley Foundation.






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