Young cannabis smokers run risk of lower IQ, report claims


Prof Terrie Moffitt, researcher: "Those who started using cannabis regularly when they were in secondary school had lost, on average, about eight IQ points"

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Young people who smoke cannabis for years run the risk of a significant and irreversible reduction in their IQ, research suggests.

The findings come from a study of around 1,000 people in New Zealand.

An international team found those who started using cannabis below the age of 18 - while their brains were still developing - suffered a drop in IQ.

A UK expert said the research might explain why people who use the drug often seem to under-achieve.

For more than 20 years researchers have followed the lives of a group of people from Dunedin in New Zealand.

They assessed them as children - before any of them had started using cannabis - and then re-interviewed them repeatedly, up to the age of 38.

Having taken into account other factors such as alcohol or tobacco dependency or other drug use, as well the number of years spent in education, they found that those who persistently used cannabis - smoking it at least four times a week year after year through their teens, 20s and, in some cases, their 30s - suffered a decline in their IQ.

The more that people smoked, the greater the loss in IQ.

Start Quote

It is such a special study that I'm fairly confident that cannabis is safe for over-18 brains, but risky for under-18 brains”

End Quote Professor Terrie Moffitt Institute of Psychiatry, King's College London

The effect was only noticed in those who started smoking cannabis as adolescents.

Researchers found that individuals who started using cannabis in adolescence and then carried on using it for years showed an average eight-point IQ decline.

Stopping or reducing cannabis use failed to fully restore the lost IQ.

The researchers, writing in the US journal Proceedings of the National Academy of Sciences, found that: "Persistent cannabis use over 20 years was associated with neuropsychological decline, and greater decline was evident for more persistent users."

"Collectively, these findings are consistent with speculation that cannabis use in adolescence, when the brain is undergoing critical development, may have neurotoxic effects."

One member of the team, Prof Terrie Moffitt of King's College London's Institute of Psychiatry, said this study could have a significant impact on our understanding of the dangers posed by cannabis use.

"This work took an amazing scientific effort. We followed almost 1,000 participants, we tested their mental abilities as kids before they ever tried cannabis, and we tested them again 25 years later after some participants became chronic users.

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There are a lot of clinical and educational anecdotal reports that cannabis users tend to be less successful in their educational achievement, marriages and occupations”

End Quote Professor Robin Murray Instuitute of Psychiatry, King's College London

"Participants were frank about their substance abuse habits because they trust our confidentiality guarantee, and 96% of the original participants stuck with the study from 1972 to today.

"It is such a special study that I'm fairly confident that cannabis is safe for over-18 brains, but risky for under-18 brains."

Robin Murray, professor of psychiatric research, also at the King's College London Institute of Psychiatry but not involved in the study, said this was an impressive piece of research.

"The Dunedin sample is probably the most intensively studied cohort in the world and therefore the data are very good.

"Although one should never be convinced by a single study, I take the findings very seriously.

"There are a lot of clinical and educational anecdotal reports that cannabis users tend to be less successful in their educational achievement, marriages and occupations.

"It is of course part of folk-lore among young people that some heavy users of cannabis - my daughter calls them stoners - seem to gradually lose their abilities and end up achieving much less than one would have anticipated. This study provides one explanation as to why this might be the case.

"I suspect that the findings are true. If and when they are replicated then it will be very important and public education campaigns should be initiated to let people know the risks."

Prof Val Curran, from the British Association for Psychopharmacology and University College London, said: "What it shows is if you are a really heavy stoner there are going to be consequences, which I think most people would accept.

"This is not occasional or recreation use."

She also cautioned that there may be another explanation, such as depression, which could result in lower IQ and cannabis use.

Marjorie Wallace, chief executive of the mental health charity SANE, said: "In a significant minority of people who are vulnerable the drug can act as a trigger to illnesses like schizophrenia which may last a lifetime."

Illicit drug use by young people has been decreasing since the mid 1990s, but the rate of decline in cannabis use throughout most of the last decade has been slow, official statistics show.


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  • rate this

    Comment number 194.

    Cannabis is bad for you and it's an uncomfortable truth to all those who've gone on for years about it being safer than cigarettes.

  • rate this

    Comment number 193.

    I sometimes indulge, I'm modest too so won't go into what I do for a living or my education. But needless to say I've done ok with my career and my life, the trouble arises (as with all drugs, alcohol being the primest example) if you are accessive with intake.
    Keep it illegal for children to obtain and smoke, but legalise and legislate adult comsumption.

  • rate this

    Comment number 192.

    Surely you have to have a pretty damn low IQ to use cannabis in the first place

  • rate this

    Comment number 191.

    "I would say you've got a pretty low IQ already if you are using drugs of any kind."

    I'd counter that by saying you've probably got a pretty low IQ if you believe any of the sensationalist anti-drug propaganda you get fed from the age of about 10 without question.

  • rate this

    Comment number 190.

    @M Picasso
    "I would say you've got a pretty low IQ already if you are using drugs of any kind."

    Where would you drawl the line on "drugs"? Meth? LSD? Weed? Alchohol? Coffee?

  • rate this

    Comment number 189.

    M Picasso
    I would say you've got a pretty low IQ already if you are using drugs of any kind.

    Well i smoked pretty much daily through my teens, and into my 20's whilst at Uni.
    My IQ test at school came out at 165, and when i retook one in my 20's it was still 165.
    Is that a low IQ?

  • rate this

    Comment number 188.

    Well this confirms observation really. Many drugs that acts on the brain can carry risks, more so in youngsters. Alcohol to name ones, TV to name another (although that is not strictly a drug!). So it seems to suggest that if you start regular use later there is reduced risk. Education and regulation would be key here. Thankfully I started at 22, and my IQ was high already, so I can afford a drop.

  • rate this

    Comment number 187.

    these researchers need to chill out a bit

  • rate this

    Comment number 186.

    It's called 'dope' for a reason. Tried it for a few weeks while at university in the first year - it just made me fall asleep. The so-called 'skunk' today is apparently much stronger.

    So, my view is that cannabis just stops you doing things you need to do and wastes so much your precious time when you could be doing real things to enjoy and make the most of your life.

    Not judging - just saying

  • rate this

    Comment number 185.

    @138 realmystery

    "Many negative ratings.....Proves my point neatly.".

    That's not particularly fair minded of you. The popularity, or otherwise, of a statement bares no relation to its veracity or integrity. Ratings systems are OK if you like being told you wonderful I suppose.

  • rate this

    Comment number 184.

    Nothing is being said about WHAT Cannabis. The gentle stuff I met in Morocco and other places in the 70's is very different from the 'Skunk' et al that's about today, some of them might as well be class A drugs.
    Anyone who is constantly bombed in their youth is going to be less can't concentrate!

  • rate this

    Comment number 183.

    If you voluntarily do something that makes you smell, gives you bad breath, gives you cancer, ruins your skin and teeth and shortens your life and messes with your brain function you are not that bright."

    Many negative ratings.....Proves my point neatly.
    Hm, doesn't alcohol do all these things too.You've proved nothing, apart from the fact that people have own choices,and will do what they like

  • rate this

    Comment number 182.

    I first tried pot aged 14 and by the time I was 17 I was smoking it every single day. I stopped 3 years ago aged 35.

    I'm not too surprised this could've affected my IQ. However I still got a decent degree, have a good job, an amazing wife and two gorgeous kids (the kids are when I stopped).

    I'm not suggesting it's harmless but it doesn't mean heavy users are doomed to failiure either.

  • rate this

    Comment number 181.

    After 4 years I’m breaking up with ex who is smoking for 15 years. Before we meet he never had serious relationship, not finished studies, and not worked. 2 years ago he started with violent outbursts. He became nasty, someone I could not imagine him to be. I get the meanest possible verbal and physical abuse. Sometimes I ask ‘has weed got into his head that much that he’s losing it?’.

  • rate this

    Comment number 180.

    They have been looking for evidence of marijuana's evils for 50 years and I've yet to hear anything that would make me believe it isn't the best option for those who want to alter their state of mind. Let's face it, people will get high. Pot is much less costly to society than alcohol. And I certainly feel safer around a bunch of potheads than a bunch of drunks.

  • Comment number 179.

    All this user's posts have been removed.Why?

  • rate this

    Comment number 178.

    Typical hilarious HYS responses to a statistical survey i.e. "this does not agree with my personal experience, therefore it is nonsense".
    Take 1 person and you will get 1 result. Take 2 you might get 2 opposing results. Take 1000 and you should get some kind of statistical significance.

    Anyway, I imagine if it is ever legalised it'll be restricted to over 18s like alcohol and tobacco.

  • rate this

    Comment number 177.

    strange how they dont hammer on about it all the time like they do with cigarettes! but there again anything that does real harm is left in place, shisha bars 100 times worse left alone, tablets to conceal drink driving left alone, they blast your brain away with a spliff or six and they leave you alone.....but there again bullys only ever pick on the law/rule abiding public

  • rate this

    Comment number 176.

    I smoked weed from being 16 until I was about 28/29, at times I was smoking massive amounts in any given day, up to an oz of bush. I also used drungs like E, Mushrooms, Speed etc but never Coke or Brown. I'd like to think it didn't do me any harm, but I lost some good friends to drugs. I was lucky I don't have an addictive tendancy and Skunk didn't arrive til the latter part of my smoking days.

  • rate this

    Comment number 175.

    In the early 1960s I attended a lecture at a prominent university where the academic shared statistics showing that cannabis was harmful to many people. I am amazed that over 50 years later bright minds are reinventing the wheel! Are there no other more important issues to be studied?

    One also wonders where a low IQ is not a motivator for people to smoke the weed!! Those with some IQ would not.


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