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.

Start Quote

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 554.

    470. A Symons
    What nonsense! As a member of a branded intellectual group, my IQ is fine. 25+ years i have been a heavy cannabis smoker.
    Look at the people who "continue" to smoke cannabis, at least 70% are of low intellect or faced disadvantages to begin with.
    Smokers do not achieve due to lack of motivation - not lowering of the IQ. If you dont use your brain...
    Equality, Paah THCism
    Enjoy life!

  • rate this

    Comment number 553.

    @552 Statistician

    The Dr would prescribe them drugs to relax - whats the difference?

  • rate this

    Comment number 552.

    What sort of message would legalising cannabis give to young people? I don't think teenagers are always great at judging the fine line between cannabis and other more dangerous drugs. There's already enough "bad" things that are legal - why do we need another one? Wouldn't it be better if people went to a doctor and got real help if they need drugs to relax?

  • rate this

    Comment number 551.

    Another man dictating what we do with our own bodies is wrong. I do applaud the article for tackling the issue rationally but its up to individuals what they do with the information.

    Heres a great article on Forbes that was recently posted regarding how preventing drug use by legal prohibition is, morally and practically, a disaster -

  • rate this

    Comment number 550.

    Under-used. You're quite right m8, sadly auntie bbc is just a propaganda channel for whoever's in power atm, spouting whatever policies their paymasters want too see aired. And the sheeple just suck it all up, as usual. This country needs a good slap :D

  • rate this

    Comment number 549.

    Stephen Jay Gould
    Carl Sagan
    Richard Feynman
    Kary Mullis
    Andrew Weil
    Sigmund Freud
    Ralph Abraham
    Timothy Leary

    Are but a few scientists who have made massive contributions not only to their respective fields, but to humanity as a whole. All of them used marijuana and / or other psychedelics. I think it's safe to say that no one here can question their IQ, especially when compared to your own!

  • rate this

    Comment number 548.

    This may go some way to explaining why older potheads all sound like rolling stones.

  • rate this

    Comment number 547.

    Loony Liberal - #425

    "I have an IQ of 259 and have been smoking cannabis since I was a toddler."

    Yeah, I don't think some of us realise what exalted company we are in on HYS - less than 0.1% of the population have and IQ of 150 or above, yet so many posting here do - aren't we lucky to have so many super-intelligent posters?

  • rate this

    Comment number 546.

    I would imagine they would be pretty relaxed about the findings or maybe a little paranoid that you are singling then out.

  • rate this

    Comment number 545.

    537 Nick. You're assuming I drive ??? You obviously miss my point.

  • rate this

    Comment number 544.

    @529: Statistician

    Because the law is currently grossly unfair, criminalising otherwise law-abiding responsible adults for recreational drug use, and not other perceived negative/risky behaviours eg. alcohol, fast food, contact sports, sitting in front of the TV every night, horse riding, I could go on. I'm not pro-drug per se, but believe that prohibition causes more problems than it solves.

  • rate this

    Comment number 543.

    That would explain EU's behaviour for marking down various people on Have Your Say, without any good reason.

    I am not saying that he or she is on drugs, or needs to be, or has ever taken them in the past.

    I hope you are feeling better today EU%-{ !

  • rate this

    Comment number 542.

    533. MoominMama

    Well done. Sort of! here is why: [1] should be where the word "it" goes, and for [2] a hyphen here isn't good form. Try a semi-colon or using a new sentence.

    You're still a bit addled. Perhaps you can stop pretending to be so superior now because I don't see any evidence to regard you as any better than a pot head: I've met several who are streets more intelligent than you are.

  • rate this

    Comment number 541.

    @530 bentyger
    This entire study is making the statement, albeit subtly, that stoners are thick. Frankly, I'm surprised the BBC ran with it as it is so obviously science used as propaganda.

    Imagine if the beeb ran an article saying: "Fat people are ugly". Oh the outrage! By the way I have no opinion on my larger brothers and sisters.

  • rate this

    Comment number 540.

    Firstly an end to comments relating to the grammar and and spelling employed in other comments? It seems trivial and little to do with the actual argument. Then we can maybe consider the fact that both parties are biased and in denial. The users do not want it to be bad for them and those who don't use are irrationally scared of it because of the illegality and media hype. Each to their own.

  • rate this

    Comment number 539.

    I smoke weed on and off and have done for nearly 10 years and I am 24, but I only drink once a year, I have a higher IQ than a lot of my mate who don't smoke weed but drink because they think these problems only effect those who smoke weed. When are drink tests going to be done? Probably never considering the public outrage it would cause to consider drink "bad". Free will I say, it's my body...

  • rate this

    Comment number 538.

    'I know this is not a very popular idea. You don't hear it too often any more … but it's the truth. I have taken drugs before and … I had a real good time. Sorry. Didn't murder anybody, didn't rape anybody, didn't rob anybody, didn't beat anybody, didn't lose – hmm – one job, laughed my ass off, and went about my day. Sorry. Now, where's my commercial?' -

    - Bill Hicks

  • rate this

    Comment number 537.

    @525. Democracy For ALL

    Have you ever gone above 70mph on the motorway?

    Are you OK with breaking that law and risking the health of yourself, your family and others?

  • rate this

    Comment number 536.


  • rate this

    Comment number 535.

    I'm 20, started smoking weed when I as 15 (took a 2 year break) been smoking it regularly. I am currently a student with a part time job. I can say that I have had no effects if anything the drug has helped me study harder, learn more and be a lot more interested in researching new topics.

    It doesn't at all damage my intelligence, I am of sound mind and able body.


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