Cannabis use in teenagers and the long-term risk to IQ

This week a report has been released illustrating a new facet of potential harm from cannabis. In a long-term study (carried out by scientists in Otago, at Duke University, USA and King’s College London), IQ and cannabis use were monitored over 25 years from age 13 to 38.

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 decline in IQ. However recurrent cannabis use was associated with an 8 point decline in IQ, comparable to that seen in early dementia. Importantly, this decline 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 the research purist, this was not a randomised study, i.e. use of cannabis was determined by the individual participants, not by random allocation – nor of course would that be ethical. This raises the obvious question whether there was bias in the type of people who used cannabis in this New Zealand cohort.

Effects on IQ could have resulted from reverse causation – i.e. cannabis used in response to underlying factors associated with likelihood of IQ to decrease with age.  The type of person using cannabis may have been more likely to use other recreational drugs that can affect brain function – including alcohol, smoking, and other psycho-active drugs of addiction. Or social or other factors may have played a role. The authors were careful to acknowledge the risk of bias of this type however said they excluded as far as possible obvious confounding reasons for their results. And validity of IQ measurements is key.

Taking the research at face value, one obvious interpretation is that the developing teenage brain is very susceptible to factors that impair concentration during a critical learning phase; or particularly prone to toxic effect on brain function of chemicals in cannabis.

If these effects are real, how could long-term effects on the brain occur? Possible explanations for the harmful effects on IQ observed by Meier and her colleagues include direct toxic effects on the brain’s natural endogenous family of cannabis receptors through intermittent overstimulation; and toxic ‘bystander’ effects on other brain pathways important for concentration and learning.

A further question is whether there are individuals at particular high risk of these adverse effects of cannabis on the brain. Recently researchers from Aberdeen have reported variation in the gene CNR1, which controls expression of natural cannabis receptors: these genetic differences can increase activity of this cannabis ‘switch’. Studies are needed to find out whether this gene may also amplify possible harmful effects of cannabis on IQ.

What next? More work is needed to understand the reasons for the falls in IQ observed with cannabis use. Whether due to cannabis or not, it is a serious public health concern if certain groups of adolescents may be prone to large decreases in intellectual function before the age of 40.  Meantime if cannabis is a likely contributory factor, given the widespread reported use of cannabis among teenagers, this study highlights the importance of well-funded and sustained public health efforts aimed at prevention and especially targeting adolescents. How will teenagers respond? A challenge to health communicators to convey successful positively framed health messages where users do no perceive current harm from cannabis in themselves or fellow users.

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• Scott Basch

  upvotes for mentioning TV in your argument

• http://www.facebook.com/izaak.wicks Izaak Wicks

  Marijuana laws are seriously biased upon the media. Mari is looked upon as a “bad drug, kills brain cells, not healthy”. As stated, it leads to an decrease in IQ around the ages below 18. This is likely due to the effects (But not harsh) carried out on your developing brain. On the other hand, the information provided isn’t exactly well told. We don’t know their full studies. Over how many people were tested? How many grams did each subject take? Variables? Aside from that, I believe cannabis should be legalized for 21+ of age.

  Alcohol, on the other hand, demands much more responsibility. It has a stronger effect on the mind and leads to lack of responsibility. Weed hardly ever causes bad effects (if any) and should be free to use, provided your body has fully developed.

  Just look at Amsterdam, where it is legal. They have a very low crime rate, and is a very well built place.

• Brandon Abell

  What is it with the Brits (edit: apparently we Yanks had something to do with it too? I am ashamed) and this kind of fake research? You’re so good at humor; so bad at science.

  For the bajillionth time: Correlation does not imply causation. At all. Ever. No matter how many times you do these “studies.”

• http://twitter.com/Sea_Drifter Curtis (Green/Orani)

  Great questions often breed even more questions. Should we stop learning about the affects of a drug that permeates our society and much of the world, simply to save money? I’d rather have more information, than less.

• http://twitter.com/DirtySketel Sam

  Science can be a slow process. More research needs to be done because research on illicit substances is inherently difficult. The scientists can’t actually give the subjects cannabis, obviously. It’s also very hard to get people to accurately report their usage. Would you rather the scientists just lied and said that they’re satisfied they know everything about cannabis now and no further study is needed?

• http://twitter.com/DirtySketel Sam

  Because as far as we know, bananas don’t have psychoactive effects and do not dramatically alter brain chemistry (in the the short term). Previous experience tells us that psycho-active and medical drugs can have long-term effects on the mind, while also telling us that the foods we eat every day don’t. Morality doesn’t come into it — just prior knowledge of other substances. Also, while IQ isn’t a perfect descriptor of every aspect of mental ability, it is generally understood to work reasonably well as a dependent variable in a study like this.

• ben

  Maybe someone should commission a study into the detremental effects that advertising, kids film/tv and the BS ‘celebrity’ culture that we have now is having on the development of the youths IQ. I would bet its worst than that of a natural, mind-opening plant.

• Richardo

  This study is nonsense but all the questionable data and theory that global warming is caused by humans is irrefutable???!! People find truth or lies where they want!