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 294.

    We do like to see things as all good or all bad, don't we? So, here we have something else that seems to do little harm in moderation (maybe even some good) but has some nasty results if we overdo it. A bit like chocolate, paracetamol, red wine, carrots, tuna, coffee, even water. You'd think we would learn quicker that things are very rarely all terrible or all marvellous!

  • rate this

    Comment number 293.

    @275.Drunken Hobo, actually cannabis alone doesnt cause lung cancer, the tobacco that its mixed with has a bigger effect.

  • rate this

    Comment number 292.

    I read this study. Researchers reasoned loss in IQ is because the brain is still developing. More youngsters smoked, greater the loss. Most relevant point is: stopping or reducing cannabis failed to restore loss. My objection: The brain develops till @ 25; so I cannot agree cannabis is safe for over-18s; this should be 25.

  • rate this

    Comment number 291.

    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. Everyone is different and for some one to get paranoid when smoking weed means you must be paranoid already or have something wrong with you. Moan something else like alcohol.

  • rate this

    Comment number 290.

    Are all of the stoners suddenly being scientific experts now?

  • rate this

    Comment number 289.

    Does this explain the recent fall in GCSE pass rates?

    What of similar studies regarding cigarettes and alcohol. I seem to remember smoking reduces brain O2 and alcohol kills brain cells. Does that mean they too reduce IQ and if so by how much in comparison to Pot?

    Could cannabis be the lesser risk, re IQ levels, and if so what advice to give teenagers?

  • rate this

    Comment number 288.

    ... yet another weight of information that suggests drugs should be considered a health issue and NOT a criminal one.

  • rate this

    Comment number 287.

    I know people that have severe arthritis and have used it very occasionally for pain relief. I've also seen first hand what can happen to people who over use this stuff and it's not pretty. I'm a little surprised that this needed investigating. It might appear to be common sense that an hallucinogenic drug may eventually cause brain impairment.

  • rate this

    Comment number 286.

    We live we die,Plenty other things in the world to kill ourselves with.The oppertunities are endless!

    Idiots take many forms.

  • rate this

    Comment number 285.

    272 perkinwellbeck. Dude, you're baked right now, aren't you?

  • rate this

    Comment number 284.

    Interestingly, the data in the study also suggest that people still regularly smoking weed at 38 years old had lower IQs to begin with.

  • rate this

    Comment number 283.

    Legalise , Regulate, Tax and control. ALL drugs.

  • rate this

    Comment number 282.

    All people who smoke cannabis must have a very very low IQ in the first place

  • rate this

    Comment number 281.

    I would rather a "lower IQ" (if this is true.........) than be dead from alcohol poisoning, cigarette poisoning (which by the way often uses radioactive fertilizer and who's industry receives support from governments all around the world), pharmaceutical poisoning, propaganda poisoning, caffeine poisoning, junk food poisoning etc

  • rate this

    Comment number 280.

    I can not believe this is still a topic of discussion! Cannabis is now known to cause paranoia and having met numerous members of the public who use the drug I can state it also robs them of their intelligence. That covers the whole spectrum of the generations that use it.

  • rate this

    Comment number 279.

    Anyone that smokes, be it cannabis or tobacco, has some deficiency, be it IQ or intelligence.

  • rate this

    Comment number 278.

    I admitt I do dabble with the drug, without trying to sound like im big headed I have a very good business which I started from scratch, im earning nice money and when I get home I enjoy a smoke to help me wind down. I also smoked it heavily between 15-21 and still managed to start my business. I do feel my mind is a bit slower than it should be but it has never held me back!

  • rate this

    Comment number 277.

    @225 papko

    "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 ;-)".

    From which I can only deduce that you do not feel that a cannabis smoker is a social outcast or a bad candidate for godfather.

  • rate this

    Comment number 276.

    I'm thinking to myself that the Government is trying to "Scare" the public by theoretical science and Television Media, as before cannabis and hemp deducts the paper industry and tax. pretty typical when they Only research the NEGATIVE side effects and not the positives of cannabis.

  • rate this

    Comment number 275.

    252 Cykogenesis
    "2. No, Cannabis does not cause lung cancer"

    That's always my favourite argument used by cannabis users. Apparently cannabis produces a magical smoke, that when inhaled directly into your lungs, doesn't cause cancer.
    Of course it does, it's burning plant matter that you're inhaling! Don't kid yourself, either accept the risk or quit.


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