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Persistent cannabis users show neuropsychological decline from childhood to midlife

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It’s been a long road from “just say no” ads to medical marijuana advocates. While non-medical use of pot is still illegal in the United States, it’s been decriminalized in a dozen states, and polls show rising public support for legalization. With growing acceptance comes the need to better understand the physical and mental effects, both positive and negative, of cannabis use.

The issue of medical marijuana involves, of course, a distinct set of risk factors that must be weighed. A 2012 study suggests there is evidence to support the use of marijuana as a “substitute for prescription opiates in the treatment of chronic pain.” Other research has found “significant potential physical and psychotropic side-effects” of medical use.

Research on longer-term recreational use also continues. Because there are a wide range of variables, however, including a user’s initial IQ, age at first use, subsequent frequency, level and duration of consumption, questions remain — in particular, whether the brain can recover after prolonged periods of abstinence.

A 2012 study from Duke University, the University of Oregon, King’s College London and the University of Otago (NZ) published in the Proceedings of the National Academy of Sciences, “Persistent Cannabis Users Show Neuropsychological Decline from Childhood to Midlife,” examined the potential relationship between the long-term use of marijuana and declines in IQ scores. The study was based on data from more than 1,037 individuals followed from birth until age 38. Cognitive tests were conducted at age 13 and again at 38; beginning at age 18, participants were regularly asked about their cannabis use. The researchers controlled for such factors as education levels, alcohol consumption and use of other drugs. Among the survey sample, 153 individuals had been diagnosed at least once for cannabis dependence and 508 total had used marijuana at some point. Some limitations are acknowledged by the authors (see at bottom of post.)

The study’s findings include:

  • Participants who had the most persistent cannabis dependence — 38 individuals, each with three or more medical diagnoses of cannabis dependence — experienced the greatest negative impact: “IQ decline was most pronounced among the most persistent cannabis-dependence group … but the effect of persistent cannabis dependence on IQ decline was not solely attributable to this group.”
  • Moreover, the “association between persistent cannabis dependence and full-scale IQ decline was still apparent after excluding the study members with 3+ cannabis-dependence diagnoses from the analysis.”
  • Those who never used cannabis showed a slight increase in IQ, while the heavier, longer-term users exhibited a loss of approximately six IQ points.
  • Neuropsychological impairment was primarily evident in attention problems and reduced processing speed and executive functioning.
  • Those who began using cannabis before age 18 were more likely to become persistent users, defined as once a week or more, and suffered the greatest decline in mental functioning — approximately eight IQ points.
  • Adult-onset cannabis users did not appear to experience IQ decline, indicating that adolescents’ developing brains could be uniquely vulnerable to early pot use.
  • Quitting pot did not completely restore mental functioning of participants who had started using cannabis as adolescents, but it could prevent additional impairment.

The authors caution that the study has a number of limitations and additional research is required to refine the results and confirm certain aspects of the findings: “Although we were able to rule out a set of plausible alternative explanations for the association between persistent cannabis use and neuropsychological functioning such as premorbid neuropsychological deficit and hard-drug and alcohol dependence among persistent cannabis users, our data cannot definitively attest to whether this association is causal.” The data were self-reported by study participants with no external source of validation. Moreover, “additional research is needed to define the parameters of use sufficient to produce neuropsychological impairment, such as the quantity, frequency, and age-of-onset of use. Our findings suggest that regular cannabis use before age 18 y predicts impairment, but others have found effects only for younger ages (10, 15).”

Keywords: drugs, addiction, mental health, youth, cognition, decriminalization

    Writer: | Last updated: August 29, 2012

    Citation: Meier, Madeline H.; Caspi, Avshalom; et al. "Persistent Cannabis Users Show Neuropsychological Decline from Childhood to Midlife," Proceedings of the National Academy of Sciences, August 2012. doi: 10.1073/pnas.1206820109.

    Analysis assignments

    Read the issue-related New York Times article titled "Marijuana Use Growing Among Teenagers."

    1. What key insights from the study should reporters be aware of as they cover these issues?

    Read the full study titled "Persistent Cannabis Users Show Neuropsychological Decline from Childhood to Midlife."

    1. What are the study's key technical term(s)? Which ones need to be put into language a lay audience can understand?
    2. Do the study’s authors put the research into context and show how they are advancing the state of knowledge about the subject? If so, what did the previous research indicate?
    3. What is the study’s research method? If there are statistical results, how did the scholars arrive at them?
    4. Evaluate the study's limitations. (For example, are there weaknesses in the study's data or research design?)
    5. How could the findings be misreported or misinterpreted by a reporter? In other words, what are the difficulties in conveying the data accurately? Give an example of a faulty headline or story lead.

    Newswriting and digital reporting assignments

    1. Write a lead, headline or nut graph based on the study.
    2. Spend 60 minutes exploring the issue by accessing sources of information other than the study. Write a lead (or headline or nut graph) based on the study but informed by the new information. Does the new information significantly change what one would write based on the study alone?
    3. Compose two Twitter messages of 140 characters or fewer accurately conveying the study’s findings to a general audience. Make sure to use appropriate hashtags.
    4. Choose several key quotations from the study and show how they would be set up and used in a brief blog post.
    5. Map out the structure for a 60-second video segment about the study. What combination of study findings and visual aids could be used?
    6. Find pictures and graphics that might run with a story about the study. If appropriate, also find two related videos to embed in an online posting. Be sure to evaluate the credibility and appropriateness of any materials you would aggregate and repurpose.

    Class discussion questions

    1. What is the study’s most important finding?
    2. Would members of the public intuitively understand the study’s findings? If not, what would be the most effective way to relate them?
    3. What kinds of knowledgeable sources you would interview to report the study in context?
    4. How could the study be “localized” and shown to have community implications?
    5. How might the study be explained through the stories of representative individuals? What kinds of people might a reporter feature to make such a story about the study come alive?
    6. What sorts of stories might be generated out of secondary information or ideas discussed in the study?


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    Phil Corn Sep 7, 2012 0:36

    Your review is a useful, brief summary of the new, very important — and very disturbing results of this research. However, I have two critical comments.

    First the study was not “from the University of Oregon” as stated in the fourth paragraph. The 10 authors hail from Duke University, King’s College London, and the University of Otago in Dunedin, New Zealand, where the field research was performed through the long-term Dunedin Study. The “editor” of the report, Michael Posner of the University of Oregon, is a member of NAS, and apparently such editorship is a requirement for publication in the PNAS by non-members.

    Secondly, in attempting to be evenhanded about purported pros and cons of cannabis, the introduction does not appropriately frame the new, and remarkable finding of this study: regular marijuana use in adolescence is found to be associated with a significant and potentially irreversible decline in IQ, as measured in adulthood!

    John Wihbey Sep 7, 2012 9:21

    Phil – Many thanks for your good note and feedback. We updated the post to reflect the true breadth of the research team; we had indeed just named the corresponding author’s institution, for brevity’s sake, but we’ll amplify that. As for your second point, we appear to express that notion fairly clearly, though your rendering is indeed more direct. Given the authors own caveats, we wanted to be especially nuanced in the write-up. Bullet points 2 and 5, for example, make the point fairly succintly. The site is meant to be a resource — a gateway to knowledge. For those reasons, we don’t mind “burying the lead,” as it were, in the name of depth and accuracy. In any case, we appreciated your note! Please come again. Regards, John Wihbey (editor)

    Marijuana use, legalization and cognitive effects: Research perspectives - Journalist's Resource Journalist's Resource: Research for Reporting, from Harvard Shorenstein Center Sep 3, 2013 14:55

    […] stakes. For example, a 2012 study published in the Proceedings of the National Academy of Sciences, “Persistent Cannabis Users Show Neuropsychological Decline from Childhood to Midlife,” found evidence that long-term use of marijuana early in life led to declines in IQ scores. But the […]

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