After adjusting for confounding factors, even heavy cannabis users had predicted IQ scores no different to those who had never tried cannabis

See also: Raw Story – Reefer madness debunked as major UK study finds marijuana does not reduce IQ in teens


Screen Shot 2016-01-14 at 8.38.27 PM.pngFrom The Guardian: Does Cannabis Really Lower Your IQ?

Whether or not using cannabis can lead to cognitive impairment is a hot topic of research and public interest. Given the extensive media attention granted to findings that suggest detrimental effects of cannabis on cognition, brain function and mental health, you would be forgiven for thinking smoking a spliff was akin to repeatedly bashing yourself over the head with a giant bong. However, since much of the work to date is cross-sectional (that is, measurements are taken only at one time in a person’s life), we cannot know whether cannabis users would have performed any differently before they started using cannabis. In short, we’re faced with a classic “chicken or egg” problem.

Cannabis use does not occur in a vacuum. And teenagers who start using cannabis from a young age will almost certainly differ from those who will never try, it or who delay until they are older. The evidence suggests that those who start using cannabis from a young age often have less stable backgrounds and more behavioural problems than their non-using peers. Teenage cannabis use also typically goes hand in hand with other drug use and risky lifestyle choices in general. The poorer cognitive performance of cannabis users may therefore result from other factors associated with cannabis use, rather than cannabis use itself. However it is of course very difficult to control for all these other factors.

To attempt to tackle these issues, along with other researchers from University College London and University of Bristol (including Suzi Gage who hosts this blog), I have been involved in a new study, with potentially surprising findings. Using data on 2235 teenagers collected as part of the “Children of the 90’s” cohort from South West England, we looked at the relationship between how many times someone reported having used cannabis by the age of 15, and their performance on an IQ test completed at the same age. Importantly the teenagers had also taken an IQ test when they were 8 years old (before any of them had used cannabis), so we could tackle the “chicken or egg” problem.

At first look our results suggested that those teenagers who had used cannabis performed worse on their teenage IQ tests, after accounting for their ‘baseline’ IQ at 8 years old. Even those who had only used cannabis a handful of times scored roughly 2 IQ points lower than those who had never tried cannabis. However, we also noted that the teenagers who had used cannabis were much more likely to have used cigarettes, alcohol and other illicit drugs- and all these factors also predicted lower teenage IQ scores. Most strikingly we saw that cannabis users were also much more likely to be tobacco cigarette smokers- 84% of those in our heaviest cannabis use group (who reported having used cannabis at least 50 times by age 15) had smoked cigarettes more than 20 times in their life, compared to just 5% of those who had never used cannabis.

When we statistically adjusted for these differences in rates of other substance use, along with other factors including childhood behavioural problems and mental health symptoms, cannabis use no longer predicted lower IQ scores. After this adjustment even our heaviest group of cannabis users had predicted IQ scores no different to those who had never tried cannabis. We also ran a similar analysis to look at the same teenager’s school GCSE grades, which they sat at age 15/16. The findings were similar to our IQ findings- while cannabis users achieved lower grades at GCSE (the equivalent of 2 grades lower on one subject), once we took account of these other related factors cannabis use no longer predicted worse school performance.

It seems therefore that there is something else about these two groups of teenagers (those who had used cannabis by age 15 and those who had not) that is responsible for the differences in IQ and school grades, rather than their cannabis use, though it’s not clear what from our study. Although cigarette smoking was identified as a potentially important factor, we clearly can’t know from this type of study whether it actually causes lower IQ and school performance, and there is little evidence elsewhere to suggest this is the case.

While this may sound like great news for those 15% of 15-24 year old Europeans who have used cannabis in the past year, the take home message is sadly not so clear cut. This is just one study from one cohort in one area of England, and as authors of the paper we are the first to acknowledge the limitations of this work, including the young age of the participants when we measured IQ, and the relatively moderate levels of cannabis use.

A well-publicised study from 2012 suggested that cannabis use starting in adolescence and persisting into mid-life “is” related to IQ decline. So how do these potentially opposing findings fit together? The key difference between the 2012 study and ours is the type of cannabis users included in the study. Our heaviest using teenagers had been using cannabis for approximately 2 years, and had used cannabis at least 50 times each (although 57% of this group reported having used cannabis at least 100 times). In the 2012 study those who showed the most dramatic IQ decline had been persistent cannabis users from adolescence until their late 30’s, and had been diagnosed with cannabis addiction at numerous points in their life. So it’s possible that cannabis addiction, rather than cannabis use per se, is related to lower IQ, or that persistent heavy cannabis use throughout your lifetime can to have these negative effects.

Our study is by no means definitive, but it does highlight that we should all be more cautious when jumping to conclusions about the harms of a drug before we have strong evidence either way. Overly forceful conclusions about the potential negative effects of cannabis are unscientific and based on an incomplete evidence base. This can lead to the unfair marginalisation of teenagers who use cannabis, which is the last thing we would want, given that this group is likely to include some of the most vulnerable in society.

Claire Mokrysz is a PhD student at University College London investigating whether teenagers are particularly susceptible to harm from cannabis and alcohol use.


5 thoughts on “After adjusting for confounding factors, even heavy cannabis users had predicted IQ scores no different to those who had never tried cannabis

  1. Marijuana myths, effects on teens probed by panel in Halifax
    Canadian Centre on Substance Abuse kicked off a 4-city tour in Halifax
    By Aly Thomson, The Canadian Press Posted: Jan 16, 2016 1:22 PM AT Last Updated: Jan 16, 2016 1:22 PM AT

    (Photo):Dr. Philip Tibbo, left, a psychiatrist, professor and researcher at Dalhousie University, talks with Dr. Selene Etches, a child and adolescent psychiatrist at the IWK Health Centre, at a forum in Halifax on Friday. (Andrew Vaughan/Canadian Press)

    How legalized marijuana could change Canadian cities
    Doctors advised kids and cannabis don’t mix
    E-cigarettes used by teens to vape cannabis, U.S. study finds

    At a Halifax skate park last summer, Philip Tibbo’s 14-year-old son was told by a group of older teenagers that marijuana is natural and no harm would come of using it.

    It’s one of many myths about cannabis circulating amongst Canadian youths today, said Tibbo, a professor at Dalhousie University’s Department of Psychiatry.

    “I asked him if many people were smoking (at the skate park) today. And he said, ‘Yes, but they’re all saying it’s harmless. That it doesn’t do anything to you’,” said Tibbo.

    “So I put on my best parental face and prevented myself from pulling the car over, pulling out my laptop and doing a presentation on it. It’s amazing that myth is out there.”

    The Canadian Centre on Substance Abuse kicked off a four-city tour in Halifax on Friday intended to debunk myths about pot and warning about the effects of the drug on young people.

    Impact on the adolescent brain

    The tour comes a month after the Liberal government’s December throne speech in which it pledged to “legalize, regulate and restrict access to marijuana.”

    Tibbo, who contributed to a report released in June about cannabis use in adolescence, said research indicates the risks are greater for teens who use marijuana because their brains are still in development.

    “The adolescent brain is going through so many more developmental processes and the whole endocannabinoid system is responsible for those processes, so then if you get regular cannabis use into that system, it can have deleterious effects down the road,” said Tibbo at a Halifax convention centre.

    “We need to get that message out.”

    Sherry Stewart, a professor at Dalhousie’s Department of Psychiatry Psychology and Neuroscience, said there are many misconceptions among youth about cannabis, including that all teenagers smoke weed.

    “In relation to what youth think, that everybody is doing it,the statistics clearly show that’s not the case,” said Stewart,
    citing the 2013 Canadian Tobacco, Alcohol and Drugs survey results for 15- to 19-year-olds.

    “Around 75 to 80 per cent of youths are not using in the past year. So that creates a social norm where youth think that everybody is using it, so why wouldn’t I do, when it fact, it’s not the majority that are doing it.”

    Calls for more research

    The report from the federally-funded agency said regular cannabis use early in life can result in behavioural and cognitive impairments, such as poor academic performance and deficits in attention, information processing and memory.

    However, all the experts noted that more research needs to be done on the effects of marijuana on youths and adults to better inform future policies.

    Sabina Abidi, a child and adolescent psychiatrist at the IWK Health Centre in Halifax, said she hears myths about marijuana every day working with youths with psychotic disorders and schizophrenia.

    “A large number of our populations of kids use cannabis, often in harmful ways,” said Abidi, who attended the panel discussion.

    “Talks like this help with our education. Already we are strategizing around how we might be able to implement some of the interventions they discussed.”

    Future panel discussions are scheduled for Feb. 3 in Toronto, Feb. 12 in Vancouver, and Feb. 22 in Ottawa.

    Liked by 1 person

  2. 3 Studies That Show Cannabis Grows Brain Cells

    The International Journal of Neuropsychopharmacology
    July 2013
    The anxiolytic effect of cannabidiol on chronically stressed mice depends on hippocampal neurogenesis: involvement of the endocannabinoid system

    Cannabidiol (CBD), the main non-psychotomimetic component of the plant Cannabis sativa, exerts therapeutically promising effects on human mental health such as inhibition of psychosis, anxiety and depression.
    However, the mechanistic bases of CBD action are unclear.
    Here we investigate the potential involvement of hippocampal neurogenesis in the anxiolytic effect of CBD in mice subjected to 14 d chronic unpredictable stress (CUS).

    How shall a natural substance which protects brains cells and even let new ones grow harm ones intelligence, please explain.

    Scientists Discover That Cannabis Reduce Brain Damage Caused By Alcohol

    Liked by 1 person

    Scientists have found that smoking weed does not make you stupid after all
    Article Comments 103

    By Christopher Ingraham January 18 at 3:00 PM Follow @_cingraham

    Elan Nelson, a spokeswoman for a marijuana retail and growing facility, answers questions amid marijuana under cultivation in northeast Denver. (AP/David Zalubowski)
    You might have heard that smoking marijuana makes you stupid.

    If you grew up in the ’80s or ’90s, that was more or less the take-home message of countless anti-drug PSAs. In more recent years, it’s a message we’ve heard — albeit in more nuanced form — from Republican candidates on the campaign trail and from marijuana opponents at the state-level.

    The contemporary version of argument can be traced to a 2012 Duke University study, which found that persistent, heavy marijuana use through adolescence and young adulthood was associated with declines in IQ.

    D.C.’s marijuana law, explained
    Play Video1:28

    As of Feb. 26, 2015 marijuana was made legal in D.C.—sort of. Here are the ins and outs of the complex pot law. (Gillian Brockell/The Washington Post)
    Other researchers have since criticized that study’s methods. A follow-up study in the same journal found that the original research failed to account for a number of confounding factors that could also affect cognitive development, such as cigarette and alcohol use, mental illness and socioeconomic status.

    Two new reports this month tackle the relationship between marijuana use and intelligence from two very different angles: One examines the life trajectories of 2,235 British teenagers between ages 8 and 16, and the other looks at the differences between American identical twin pairs in which one twin uses marijuana and the other does not.

    Despite vastly different methods, the studies reach the same conclusion: They found no evidence that adolescent marijuana use leads to a decline in intelligence.

    [These are the states that could legalize pot next]

    I wrote about the study of British teenagers before, when it was still a working paper. It has been peer-reviewed and accepted for publication, and its findings still stand: After adjusting for a range of confounding factors, such as maternal health, mental health and other substance use, the researchers found that “cannabis use by the age of 15 did not predict either lower teenage IQ scores or poorer educational performance. These findings therefore suggest that cannabis use at the modest levels used by this sample of teenagers is not by itself causally related to cognitive impairment.”

    They did find, though, a distinct relationship between cigarette use and poor educational performance, which is in line with what other research has found. The researchers did not find a robust link between cigarette use and IQ.

    The authors of this study stress that their results don’t necessarily invalidate the findings of the 2012 Duke University paper. That paper focused on persistent heavy use over a long period of time, while this study looked only at low to moderate levels of adolescent use. “While persistent cannabis dependence may be linked to declining IQ across a person’s lifetime,” the authors write, “teenage cannabis use alone does not appear to predict worse IQ outcomes in adolescents.”

    But the researchers in the study of American twins tackle the Duke University findings head-on. Examining the life trajectories of twin pairs in which one uses marijuana while the other doesn’t, they found that those who used marijuana didn’t experience consistently greater cognitive deficits than the others.

    Identical twin comparisons are a powerful tool for this kind of analysis, because their genetic makeup is nearly identical and their early home environment is consistent. This automatically controls for a lot of the confounding factors that can make sussing out causality difficult.

    The twin data “fails to support the implication by Meier et. al. [the authors of the Duke study] that marijuana exposure in adolescence causes neurocognitive decline,” the study concludes. The numbers suggest, on the contrary, that “children who are predisposed to intellectual stagnation in middle school are on a trajectory for future marijuana use.” In other words, rather than marijuana making kids less intelligent, it may be that kids who are not as smart or who perform poorly in school are more inclined to try marijuana at some point in their lives.

    Also, if marijuana use were responsible for cognitive decline, you might expect to find that the more marijuana a person smokes, the less intelligent they become. But this paper found that heavier marijuana use was not associated with greater decreases in IQ.

    None of this is to say, though, that you can smoke all the weed you want and not have to worry about negative outcomes. There are any number of negative physical and mental health outcomes linked to marijuana use — especially heavy use. Some research suggests that heavy marijuana use may increase the risk of psychosis or suicide. These risks are further compounded among people who start using marijuana early in their lives. And people who use heavily or start at an early age are at a high risk for cannabis-use disorder, a form of drug dependency.

    Marijuana is a drug. And just like any other drug — alcohol, nicotine, caffeine — there are risks and benefits associated with use. But exaggerating the extent of those risks and benefits won’t help create smarter policies. For proof of this, simply review the history of the drug war.

    More from Wonkblog:

    Legal weed having little effect on teen marijuana use, federal data show

    Why Bernie Sanders’ marijuana proposal would be a big deal

    These ‘marijuana goggles’ are supposed to make you feel stoned



    What’s the impact of new marijuana laws? The data so far
    February 3, 2016
    Wolters Kluwer Health
    How has new legislation affected marijuana use in the United States? The best available data suggest that marijuana use is increasing in adults but not teens, with a decrease in marijuana-related arrests but an increase in treatment admissions, according to researchers.


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