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 494.

    Were they stoned when they did the IQ tests? Most probably. Also, how do you measure occupational success? By the amount of money you make? Because as anybody who's had their third eye squeegied on a regular basis will tell you will tell you money is no measure of success.

  • rate this

    Comment number 493.

    All those people frowning on Cannabis use.... you ever had a beer ? Well guess what ? Alcohol is a drug and its much worse then cannabis, go into any town centre on a Saturday night or into any A&E and you'll see the effects and social costs of Alcohol which is a widely available and advertised drug.. The hypocrisy stinks and no i'm not a cannabis user though i was when i was young.

  • rate this

    Comment number 492.

    Who wants a prescribed life? These studies are offered as information, but don't be mistaken: they are designed to modify behaviour. So who gets to decide how a person should exercise their free-will and under what criteria.

    In the long run, such a pre-determined lifestyle is no life at all.

  • rate this

    Comment number 491.

    Looking at the spellings and grammar of a lot of the pro-cannabis comments, looks like there's no need for the research - it's self evident.

  • rate this

    Comment number 490.

    the reason the study is so negative i believe is because the new zealand pot is not having enough sun ....let people choose to smoke or not freely and hey tax it more revenue why not? plus if you look around i ll say 90% of kids did try it why make such a big deal?

  • rate this

    Comment number 489.

    "cannabis smokers risk lower I.Q. ..."!

    Not being funny, but drugs are bad and people should know that, and this has nothing to do with cannabis... as people who take drugs for fun may already have a lower I.Q... not to mention a lower E.Q, to resist the urge to take them in the first place!

    Maybe they thought It'll give them a higher I.Q... as aren't drugs meant to give people the illusion?!

  • rate this

    Comment number 488.

    Wowwww this is like some kind of heavy reeseach dudes

  • rate this

    Comment number 487.

    Another study says that IQ tests do not accurately measure anyones true IQ, let alone anyone who take mind altering drugs !

    Also this study just says it CAN drop the "IQ" of a developing brain ! A point the anti drug lobby seem to overlook !

  • rate this

    Comment number 486.

    480. casualobserver84
    "I bet they had alot of fun "researching" into this before coming to an obvious conclusion!"
    I bet you haven't got a clue about scientific research but love to comment anyway!

  • rate this

    Comment number 485.

    your never going to convince a drug user that its bad for them, but like others have said, we should feel sorry for them that their lives must be pretty bad to have to use drugs in the first place.

    What ever don't legalise the stuff, can you imagine the carnage on the roads?

  • rate this

    Comment number 484.

    Quote: "I'm 26 and been smoking weed since I was 17 and I can say there nothing wrong with my brain I'm hard working and have a house and a family of my own and bet I'm smarter than half of the people that leave comments on here."

    You you don't seem able to punctuate your sentences!

  • rate this

    Comment number 483.

    @436. DrJG

    This is taken directly from the paper: full scale IQ age 7-13
    for those who never used: 99.84, used regularly at 3+ waves: 96.00
    So those who developed long lasting dependency scored much lower pre exposure
    Also group that developed to be smokers is mostly male.

  • rate this

    Comment number 482.

    I survive that what the world citizen is told them don't happen and impossible to accomplish. I know the real world and tell you there is no one that can hold the truth given the knowledge promoted by any government. Banned are too many chemicals that benefit and promote a better quality of life. Pot is just one of them. Nations are tyrannical, abuse the living controlling lives and minds.

  • rate this

    Comment number 481.

    Let's face it folks, we all know what the bottom line is.

    Legalize, regulate and tax.

    But no marketing!

    All those in favour say "Aye"

  • rate this

    Comment number 480.

    I bet they had alot of fun "researching" into this before coming to an obvious conclusion!

  • rate this

    Comment number 479.

    According to, smoking -or even second hand smoking- tobacco itself runs the risk of lower IQs in under 18s. Did this smokers in this study use tobacco in their joints?

  • rate this

    Comment number 478.

    sad news for the kids.
    but good news for those who are contemplating using dope.
    could we now have stricter legislation on grow shops legally selling all the stuff needed to grow it.

  • rate this

    Comment number 477.

    Oh it's a vicious circle! To start in the first place you've gotta be pretty stupid, and so it seems it's downhill from there...

  • rate this

    Comment number 476.

    Ah, the hypocrisy of marijuana prohibition while other, potentially fatal ways of getting inebriated are legalised–and often endorsed/used–by the very same politicians that look to keep Mary Jane in the naughty corner.

    It would be refreshing to see an article on the study of the detrimental effects of alcohol on the user and the impact it has on society when compared to marijuana.

  • rate this

    Comment number 475.

    1) Prof Val Curran said "This is not occasional or recreation use". I say: a) What other sort of Cannabis use is there? b) The article suggests it's users of various frequencies anyway.
    2) How come we haven't heard a peep from Prof. David Nutt about this?
    3) How come Nutt didn't do research that made these findings?

    This damage must be atop the 10pts deficit needed to smoke in the first place.


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