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"

Related Stories

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.


More on This Story

Related Stories

The BBC is not responsible for the content of external Internet sites


This entry is now closed for comments

Jump to comments pagination
  • rate this

    Comment number 214.

    i wonder if dave was far outman when he let ATOS sponsor the comming games? talk about a bad trip,IQ just made single figures on that decision,still it should go down well at the local facist private members club.

  • rate this

    Comment number 213.

    A thousand participants in New Zealand over 40 years... not really compelling evidence is it

    And no Government will legalise cannabis as they get far more money from confiscated money and property from drug dealers etc etc than they can ever make in taxes

  • rate this

    Comment number 212.

    I smoke Cannabis every day - I'm in receipt of very generous benefits - all of which is spent on getting wasted (I thank Iain Duncan Smith for my MASSIVE BENEFIT RAISE) I have no intention of working ever. In fact I'm too wasted to bother to work. I do however, have every intention of getting high until I die - & you mugs who pay taxes are funding my lifestyle choice. Lower IQ? You be the judge.

  • rate this

    Comment number 211.

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

    No dear, that is because they no longer give a monkeys about the narrow definition of achievement that serves the establishment and the promotion of individuals as 'experts'.

    It is pretty obvious that if you do any drug when still growing then it will be detrimental - to a point - expert am I

  • rate this

    Comment number 210.

    I say legalise it and tax it and sell it in colourful little packets in Sansbury's. Then people wouldn't have to buy it from Criminals
    They don't have to buy it from criminals now. They choose to.

  • rate this

    Comment number 209.

    ‎From the study itself: "The study’s results must be interpreted in the context of its
    For example, there may be some unknown “third” variable that
    could account for the findings." Like socioeconomic status which is NOT controlled for in this study. So headline should be rather "There is a link between cannabis use and IQ decline although no casual relation can be proven"

  • rate this

    Comment number 208.

    I just wonder why people think the alternative to cannabis not being illegal is to 'regulate it'? So the government can burden people with another TAX? Resist the DELPHI technique folks - that's where you think you have come up with a great solution to a problem but actually that's the solution they already came up with and slowly drip out so you accept that idea later. Think for yourselves!

  • rate this

    Comment number 207.

    "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."

    Has anybody argued that drinking too much alcohol does anything other than lower your IQ as well as all the other damage it does? Arguing poison a is Ok because it is not as bad as poison b is not very clever.

  • rate this

    Comment number 206.

    Proof....perhaps, that some will alledge why we have such captains of industry, finance & goverment running things.......

  • rate this

    Comment number 205.

    Its funny how some people agree with a study if it backs their views, yet, they fight it, if it goes against them.

  • rate this

    Comment number 204.

    Anyone who in their student days had the misfortune to share a flat with regular cannabis users knows first hand that the above story is absolutely true!

    Stereotypes are often grounded in fact & who can't remember meeting real life versions of 'Neil the Hippy'?

    Just as 'Old Boxers' often sound the same due to cumulative brain damage, so do heavy cannabis users except they are far more tedious!

  • rate this

    Comment number 203.

    Watch a movie called "Super High Me", it's a documentary how a smoker smokes cannabis for 30 days and how it affected him, if you need more proof, watch the movie.

  • rate this

    Comment number 202.

    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.
    Nobody saying it's good for you, it's about choice and making your own decisions in life

  • rate this

    Comment number 201.

    what is the difference for them to lose couple of points on IQ? I guess you don't care much if it is already on the lower side.

  • rate this

    Comment number 200.

    The scariest thing about smoking weed is remembering where you stashed it, if it was that bad youd never find it again. Chill out and smoke a fat one.

  • rate this

    Comment number 199.


    Was the article about you Kathreina? Nope, thought not...

    I love the way people's ego-centricity leads them to use their single example of themselves to disprove a general 'rule'.

  • rate this

    Comment number 198.

    The posting appears to stick entirely to the facts assuming Oz speaks the truth about his IQ. So why the downratings? Could it be a Pavlovian response to an old chestnut (forgive the mixed metaphors). I'll take Oz's view over the unthinkers on this one. I'm getting drawn to posting insults like some of unthinkers but I'll wait for now.

  • rate this

    Comment number 197.

    Isn't IQ meant to diminish/decline with age anyway - according to a bunch of 'other scientists'???

  • rate this

    Comment number 196.

    Balloon Rake
    People like you should be locked up for being so boring, drink is the main stream problem in britain, the ammount of deaths that are caused by alcohol is into its thousands per year, where as weed has NEVER caused a death! i dont smoke or drink but to point the finger at a drug that is far less harmful than another drug that is widely available is a joke. Ban Alcohol & Tobacco first!

  • rate this

    Comment number 195.

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

    I wish I was an expert so that I could say things like this and still command some authority and get quoted in articles and stuff. Presumably "seem to underachieve" means that we don't know for definite that they do?


Page 38 of 48


More Health stories



BBC © 2014 The BBC is not responsible for the content of external sites. Read more.

This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.