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.
     

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