Study reveals negative long-term effects of heavy cannabis use on brain function and behavior

New study in Biological Psychiatry: Cognitive Neuroscience and Neuroimaging looks at the effects of heavy cannabis use on brain function and behavior

Young people with cannabis dependence have altered brain function that may be the source of emotional disturbances and increased psychosis risk that are associated with cannabis abuse, according to a new study published in Biological Psychiatry: Cognitive Neuroscience and Neuroimaging. The alterations were most pronounced in people who started using cannabis at a young age. The findings reveal potential negative long-term effects of heavy cannabis use on brain function and behavior, which remain largely unknown despite the drug's wide use and efforts to legalize the substance.

The study, by Drs. Peter Manza, Dardo Tomasi, and Nora Volkow of the National Institute on Alcohol Abuse and Alcoholism, Bethesda, Maryland, assessed resting brain activity data from the Human Connectome Project of 441 young adults, and compared a smaller set of 30 people who met criteria for cannabis abuse with 30 controls. People with heavy cannabis use had abnormally high connectivity in brain regions important for reward processing and habit formation. The same regions have also been pegged in the development of psychosis in previous research.

"These brain imaging data provide a link between changes in brain systems involved in reward and psychopathology and chronic cannabis abuse, suggesting a mechanism by which heavy use of this popular drug may lead to depression and other even more severe forms of mental illness," said Dr. Cameron Carter, Editor of Biological Psychiatry: Cognitive Neuroscience and Neuroimaging.

The brain alterations were also associated with heightened feelings of negative emotionality, especially alienation, where one feels a sense of hostility or rejection from others. The link points to a potential biological mechanism for why feelings of alienation are often profoundly increased in people with cannabis dependence.

"Interestingly, the hyperconnectivity was strongest in the individuals who began using cannabis in early adolescence," said Dr. Manza, which lines up with reports of a higher risk of psychiatric problems when cannabis use begins early in life. Adolescence is a critical period of brain development, making early use of cannabis particularly detrimental. According to Dr. Manza, the measurement of resting brain activity is a relatively easy and non-invasive procedure, so the approach could be a useful measure for tracking the development of psychiatric symptoms with cannabis use.


  1. tyler durden tyler durden United States says:

    The problem with this study is that they didn't explore external factors in the patients lives that lead to cannabis use/abuse at an early age. What leads a youth to begin using cannabis at an early age to begin with? Are they struggling socially? Do they experience loneliness and isolation due to family problems or social problems and then turn to the outcasts and indulge in cannabis for acceptance? Correlation in this study doesn't beget causation unless they can account for the external factors that lead to early use. If you want to stop kids from using cannabis it should be regulated like alcohol first, then the factors leading to early use should be examined. It's hard to believe that the small sample size of 30 early and heavy users are well adjusted kids with good family and social lives who just somehow found a bag of grass on the sidewalk and became heavy users. Let's try again. Also, nobody under 21 should use cannabis unless it is to treat a medical condition. We need a fair look that explores more than just one factor.

  2. heddy rain heddy rain United States says:

    I agree, one study with 30 kids? These were all top of the class kids, no problems, but cannabis took them over. This should be a PSA. No child should ever have access to any drug, unless it is prescribed, and /or they are of legal age to use same as alcohol.

  3. Drew Edwards Drew Edwards United States says:

    Nora Volkow is the top neuroscientist in the US, and the Director of NIDA and former lead researcher at the national PET scan lab in Brookhaven. All covariants were addressed through regressional analysis and odds ratio estimates are statisically significant. This methodilogy is sound. If you dont like the small sample size try this: A recent NIDA-supported a longitudinal study conducted by researchers from Duke University. The team collected data from the Dunedin Multidisciplinary Health and Development Study that has followed 1,000 New Zealanders born in 1972. Participants were interviewed at 18, 21, 26, 32 and 38 and also underwent neuropsychological testing at ages 13 and 38.   The results were shocking. Persistent marijuana use was linked to a significant decline in IQ, (1/2 standard deviation) even after controlling for educational differences and other covariates. The most persistent users, experienced a drop in neuropsychological functioning equivalent to about six IQ points others a much as 8 points. Participants with more persistent cannabis dependence generally showed greater neuropsychological impairment.  Inspection of the means suggests that the highest impairment was found in domains of executive functioning and processing speed. To test whether impairment was relatively greater for certain domains, cannabis-associated neuropsychological impairment was compared across the four Wechsler Adult Intelligence Scale-IV (WAIS-IV) indexes (i.e., working memory index, processing speed index, perceptual reasoning index, and verbal comprehension index), which share psychometric properties. The results showed that associations between persistent cannabis dependence and the four WAIS-IV indexes could be equated without a deterioration in model fit (Δχ2 = 2.13, df = 3, P = 0.55). This suggests that impairment was not significantly different across neuropsychological domains. Average IQ is 100, or 50th percentile, marijuana use over time dropped individuals who were at the 50th to the 29th percentile. Meaning that 71% of Americans are now smarter than you.

The opinions expressed here are the views of the writer and do not necessarily reflect the views and opinions of News Medical.
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