Teenagers, IQ risk and cannabis: cause and effect, bias or other explanations?

@HealthMed My first contact with study of the science of cannabis was as a medical student in Aberdeen undertaking a summer project in pharmacology: a new researcher, Roger Pertwee, was interested in the effects of bioactive ingredients of cannabis on brain regulation of temperature and muscle function.  Research since then by Pertwee and many others has identified the importance of endogenous chemicals, endocannabinoids, that are part of normal functioning of the brain and other parts of the body, for example in modulating mood and stimulating appetite. Abnormalities in endocannabinoid pathways have also been implicated in a wide range of medical conditions, including inflammation, obesity, cancer, cardiovascular disease as well as mood disorders. 

Now this week we have a report illustrating a new facet of potential harm from cannabis. In a long-term follow up study by scientists in Otago, at Duke University, USA and King's College London, lead by Madeline Meier (Duke), IQ and reported cannabis use was monitored over the 25 years from age 13 to 38 in the Dunedin Birth Cohort. Of particular interest, their study of over 1000 subjects included teenagers in whom IQ was checked before any cannabis exposure. Around 1 in 7 reported being regular cannabis users, 1 in 20 doing so at least weekly before the age of 18.

What did the investigators find? With no cannabis history, there was a small fall in IQ. However recurrent cannabis use was associated with an 8 point decline in IQ, comparable to that seen in early dementia. Importantly, this decrease in IQ was particularly marked when cannabis use began during teenage years. A further concern was that stopping cannabis use did not lead to recovery of the IQ loss. Commentary on the results has ranged from concluding that cannabis is harmful in teenagers but safe in adults, to more cautious notes that adolescent brains appeared more vulnerable to cannabis, without providing carte blanche for longer term safety of cannabis use in adults …

Are these fair interpretations? For further discussion of this long-term, prospective observational study, important caveats in its interpretation, and its potential implications, see my discussion posted on The Independent blogs site.