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 234.

    @jamdoonsouth - is that 16.5?
    @King Red - you are wrong - death has been caused by Cannabis - usually car accidents, shootings, knifings - oh yes the people taking it themselves don't die - others do dumbnut! Also contributes to mental illness - making you pay more tax to treat people in hospital and community and raising the risk very everyone else. Just because it didn't affect you......

  • rate this

    Comment number 233.

    We are surrounded by drugs all the time: alcohol, caffeine, and so on. The problem is that some of these (alcohol) were capitalized on by big businesses before the hard solid evidence of its destructiveness became known. There does not seem to be the will to deal with alcohol, but I think that where one can, it is a good idea to stop more drugs and their destructiveness from prevailing more.

  • rate this

    Comment number 232.

    Some of the greatest thinkers throughout history have been users of cannabis and other recreational drugs. A world without drugs would be a world without good music, vast variations of art, and inspiring cinema. If you enjoy listening to One Direction and watching tacky spoof films then fair play, but it's not my bag thanks.

  • rate this

    Comment number 231.

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

    Do I need to point out how ridiculous that sounds? You just highlighted that fact that you don't need to be on drugs to come accross as stupid and ignorant.

  • rate this

    Comment number 230.

    Regardless of whether it's right or wrong, it's illegal to use cannabis. Now tell me - are users so spectacularly arrogant as to believe they're above the law.
    What a poor argument. It used too be against the law too be gay,it used too be against the law too have shops open on a Sunday.
    The law is an ass.
    It's arrogance to think that we don't have the right too stand up for what we want.

  • rate this

    Comment number 229.

    Presumably "seem to underachieve" means that we don't know for definite that they do?"

    And what does underachieve mean anyway? Someone who 'should' have been a 'success' may be perfectly happy earning a few bob (with no real prospect of making anything more than a living) as, say, a musician and be far more content with their lot than an 'achiever'.

  • rate this

    Comment number 228.

    Maybe people with lower IQ's are more likely to use cannabis?

  • rate this

    Comment number 227.

    Why do we constantly get the bad news stories when it comes to cannabis. Has there been any studies in to the benefits of medical marijuana, such as the potential to cure some skin cancers? No, because the drug companies don't want those studies carried out, and it would be too much of a positive message for the media. Abuse of marijuana will be no less damaging than alcoholism. Hardly news.

  • rate this

    Comment number 226.

    Young people who smoke cannabis for years also run the risk of lung cancer, just like smoking cigarettes. smoking is not cool and that includes smoking cannabis. End of.

  • rate this

    Comment number 225.

    Thats no surprise to me , i know loads of Kiwi Cannabis users and thye are all dopey , vacant types , including Best man at our wedding , and the Godfather of our son , more so than most actually ;-)

  • rate this

    Comment number 224.

    @164 thrill_vermillion. Yes, it is illegal, its also *not reccomended* to drink alcahol. But when they made it illegal it opened up a billion pound industry. That the government can't control. Tell me, were you the kid at school that got his lunch money stolen? Because the government kind of take our lunch money everyday. Don't they?

  • rate this

    Comment number 223.

    "Cannabis" is a generic term nowadays, like "alcohol"

    There's a world of difference between a pint of lager and a pint of whisky

    Cannabis is the same
    There are lager strength spliffs and there are some seriously squeaky whisky strength ones, skunk etc

  • rate this

    Comment number 222.

    192. dbgvwwz
    'Surely you have to have a pretty damn low IQ to use cannabis in the first place'
    I know some VERY clever people who like a joint. Also, some of us get pain relief for things like arthritis from it. MODERATION is important, especially nowadays with so much Cannabis being cultivated for great strength.
    I stopped smoking it myself, it triggers anxiety now, but not in the past.

  • rate this

    Comment number 221.

    One of the names it used to called before it was 'cool' is 'dope'.
    I think that is a word that describes the addictive cannabis smoker ( oh sorry, it not addictive is it? ) and affirms the study ( and what everyone with common sense knew anyway)

  • rate this

    Comment number 220.

    legalise it and tax it

  • rate this

    Comment number 219.

    Having seen the effects in a number of people the most apparent one is paranoia, this in some cases can cause the victim to question most things and slow their processes down thus reducing their score.

  • rate this

    Comment number 218.

    I think its about time high flyers were more open about their own cannabis use.I would like to see a high profile campaign to put more pressure on legalisation.

  • rate this

    Comment number 217.

    There does seem to be a somewhat romanticised view of cannabis. Simply, it is a mind-altering drug so it will have side effects, just like every other drug - cannabis is not special.
    People often bring up how alcohol & tobacco are abused - so I'd say that's a pretty solid argument against legalising it! We seem to abuse things more when they're legal; perhaps legality makes people assume safety.

  • Comment number 216.

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

  • rate this

    Comment number 215.

    Im not sure if cannabis use lowers IQ as much as watching zombie nation TV programmes. Yes I mean you too BBC with the mind numbing Strictly. Would be interesting to see the results of research into that.


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