Heavy Cannabis Use Linked to Psychosis and Cognitive Deficits

Research finds significant correlations between early, chronic, heavy use of cannabis and psychosis-related phenomena and cognitive deficits.


A recent study published in Psychological Medicine explores the potential effects of early, chronic, and heavy cannabis use. The authors examine the relationship between heavy use of cannabis and psychosis-related psychological phenomena, as well as cognitive dysfunction.

Comparing statistical data between “case” and “control” groups, for heavy cannabis users, they found significantly increased presence of psychosis-related phenomena and significantly reduced cognitive functioning across several subdomains. Limitations in the diversity of the sample, among other issues, point to the need for further research.

“Cannabis is one of the most commonly used psychoactive substances worldwide. In some areas studied, over the past two decades, there have been significant changes in the patterns of cannabis use characterized by increased prevalence of use among adults, decreased perception of harm among adolescents, and unintended prenatal and childhood exposure,” write the authors.
“Despite a decline in the perceived harm of cannabis, several adverse health consequences, including neuropsychiatric sequelae, have been linked to regular and heavy cannabis use. The rapidly evolving landscape of cannabis use in the background of changing medical and recreational marijuana laws necessitates clarifying the existing uncertainties regarding the causal impact of cannabis exposure on these adverse health outcomes.”

Previous research has found associations between marijuana use and the onset of psychosis, as well as other conditions, like depressive and anxiety symptoms. Some have pointed out, of course, that dosage and patterns of use are important factors here, complicating the issue of marijuana or cannabis being inherently dangerous. Individuals vary in their response to marijuana, with some being more prone to heavy, habitual use. Additionally, not all research has confirmed these associations, although negative studies have been critiqued.

The current study looks to understand the psychological effects of heavy, chronic, and early cannabis use. In particular, the researchers were interested in potential links to psychosis-related phenomena as well as cognitive effects related to memory, attention et al. The authors state that previous research into these relationships has been marred by insufficient attention paid to comorbid, potentially confounding factors, such as multi-drug exposure, initial age of use, duration of exposure, and more.

Attempting to control for these variables, the researchers studied a specific community of people “forbidden from using other substances, including tobacco and alcohol.” This community uses cannabis for purposes of “enlightenment, social bonding, medicinal uses, and rituals.” Use begins early for community members, sometimes in utero, and is both “heavy” and “chronic.”

The study’s participants were mostly of African ancestry, English-speaking, and were spread out geographically within the country where the study was performed. A control group was recruited with similar demographics related to education, age, gender, and ethnicity. The participants varied in their occupation, and most were male—14 of 15 in the case of the sample, 10 out of 12 in the control group.

Various psychometric scales were used to gather data, such as the Scale Assessing Lifetime Cannabis Use (SALCU), the Schizotypal Personality Questionnaire (SPQ), and several batteries to measure cognitive functioning. These data were analyzed statistically using SPSS.

10 out of 15 participants in the cannabis-smoking group reported initiating use before age 18. The same number reported smoking cannabis every day for the previous month, while the remaining 5 reported smoking most days.

Their mean score on the Schizotypal Personality Questionnaire (SPQ) measuring psychosis-related phenomena was 24, compared to 13 for the control group. This was a statistically significant finding (p = .03). Specifically, the “case” group exhibited “odd beliefs, and magical thinking, unusual perceptual experience, and odd and eccentric behavior” according to the scale.

In terms of cognitive functioning, the case group performed worse than the control group on all measures:

“[…] moderate to large effect sizes for between-group differences were noted in the Detection Test (attention), Identification Test (psychomotor speed), One Back Test (working memory), Set Shifting Test (cognitive flexibility), Chase Test (visuospatial processing), and Shopping List Test (memory).”

Similar effects were found for verbal learning and total immediate recall, while no differences emerged for delayed recall.

Adding to the evidence, the study’s authors included data from 3 of the case group members’ siblings, hoping to control for confounding variables such as genes, upbringing, socioeconomic status, nutrition, and more. They found that case group members scored higher on the SPQ measure than their siblings, while the siblings’ scores were statistically similar to control group scores.

Likewise, the siblings performed better on verbal memory and attention tests.

The authors do caution in the discussion section, however, that only a “small minority” of those exposed to cannabis appear to develop psychosis. In addition, the psychological phenomena measured by the SQP questionnaire do not necessarily suggest full-blown psychosis and/or schizophrenia.

The authors note several further limitations to the study results.

Because the research was primarily cross-sectional rather than longitudinal, it is impossible to conclusively determine whether cannabis was the cause of these psychological differences or whether they pre-existed the long-term, heavy use of cannabis.

Limited longitudinal testing was done twice, 6 years apart, which confirmed previous findings, but only 4 participants were involved in these later tests.

The authors also state that the unique (and small) sample size used in the study—97% of participants were of “partial or total African descent,” and most were male—makes generalizing to other populations difficult.

The authors conclude, suggesting the need for further research:

“The findings of this study suggest that early, chronic, heavy, and, importantly, isolated cannabis exposure is associated with attenuated psychosis symptoms and cognitive dysfunction. The findings in this unique but small sample warrant replication in a larger and longitudinal study of this or a similar population to more fully understand the cognitive and behavioral effects of chronic, heavy, early cannabinoid exposure without the confounding effects of other drugs.”



D’Souza, D. C., Ganesh, S., Cortes-Briones, J., Campbell, M. H., & Emmanuel, M. K. (October 01, 2020). Characterizing psychosis-relevant phenomena and cognitive function in a unique population with isolated, chronic, and very heavy cannabis exposure. Psychological Medicine, 50(14), 2452-2459. (Link)

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Micah Ingle, PhD
Micah is part-time faculty in psychology at Point Park University. He holds a Ph.D. in Psychology: Consciousness and Society from the University of West Georgia. His interests include humanistic, critical, and liberation psychologies. He has published work on empathy, individualism, group therapy, and critical masculinities. Micah has served on the executive boards of Division 32 of the American Psychological Association (Society for Humanistic Psychology) as well as Division 24 (Society for Theoretical and Philosophical Psychology). His current research focuses on critiques of the western individualizing medical model, as well as cultivating alternatives via humanities-oriented group and community work.


  1. The question is whether these cognitive deficits were around before or after cannabis use. Sometimes recreational drugs are consumed to make a miserable experience less so. Such individuals will seldom to never stop using them if their mental functioning isn’t simultaneously improved.

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      • Stevie, you seem to suffer from cognitive dissonance. Any psychoactive drug taken chronically and as you say with “heavy use” will cause cognitive damage. Be it cannabis or an antipsychotic.

        You put psychosis in quotes, it seems you’ve never experienced a real psychosis. Refrain from putting quotes around it, unless you know what you are talking about. I think you don’t.

        A real psychosis is like being in a dream, there is no “ego” to guide you. I’m sure you know how crazy dreams can be?

        Just like dreaming, a psychosis is a necessary escape.

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          • You don’t have any first-hand knowledge of the damage antipsychotics have. I do. So, do me a favor, and stop putting quotes around psychosis. It’s a silly game everyone here plays. Everyone posting here is convinced they were misdiagnosed, from a diagnosis that doesn’t exist. It’s what everybody does, it’s natural. Everybody lies. The truth is too painful for most.

            Here’s an example of a bad day:


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          • The point of putting quotation marks around “psychosis” is not to deny such experience exists, but to take away the implication that “psychosis” is a “thing” that can be viewed as a unity. It’s an experience that has many potential causes, which have been lumped together into one purported entity for purposes of making money and saving the trouble of having to seek out the actual causes.

            Tell you what – I won’t correct your language in the future, and you don’t correct mine. If you wonder why I’m using a particular convention, maybe you could ask instead of assuming and telling me my own reasons? It would seem a lot more polite to me.

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          • Do you think politeness will get you anywhere? You put quotes around “psychosis”, “schizophrenia”, “mental illness”, because you haven’t got the foggiest idea of what any of these things REALLY mean. I do. Being polite will get you gassed in the end. You have to at least get a grip on the concept of deception, it’s been around for a very long time. Playing along with their game will not help you.

            I’m not trying to correct your language, simply pointing out you are “the blind leading the blind”.

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          • I fully acknowledge not knowing what these things mean, mostly because I don’t think they mean anything specific, but are simply descriptive syndromes that don’t really hang together in terms of cause.

            As for politeness, I find it perfectly viable to be both polite and firm. Of course, there are people who won’t respond to politeness, but that’s no reason to toss it out in every single case. There are plenty of people who will listen better if the person speaking doesn’t start off being presumptuous and offensive. I save that for those who don’t seem to be capable of hearing things presented more rationally. Which, admittedly, is a lot more common in this particular field of endeavor.

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          • I have a hard time thinking there can be a unitary cause for psychosis, since we know that it can be caused by things as disparate as several days of no sleep and excessive stimulants and childhood sexual molestation and living in an urban environment. But I’m interested to hear – what does Jung say is the cause? I have read some of Jung’s work but this one has not come to my attention. And any explanation that gets away from brain-blaming seems very worth considering to me.

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        • A lot of us know exactly what terms like psychosis, mental illness, or schizophrenia imply (there I put it out of quotes). Yes, we know people can be delusional or hear voices. But it is psychiatrists who label people as schizophrenics when that happens. We know people can be depressed or euphoric (sometimes due to psychiatry’s own drugs), but it is psychiatrists who label them as major depressives or bipolars. We know people may have intrusive thoughts or not be able to concentrate in class. But it is psychiatrists who label them as OCD or ADHD individuals. We know people can be stern and volatile. But it is psychiatrists who label them as borderline. We know people have may have several problems, but it is psychiatrists who end up labelling them with a gamut of labels that destroys them fairly often. We know all these things can be associated with extreme distress to the point that a person can’t function and doesn’t know what else to do (which is where people end up in psychiatry). There are reasons for everyone’s behaviour and so be it.

          And it is not that we were misdiagnosed from a diagnosis that doesn’t exist (of course it does, it’s applied daily to people when they end up at the doorstep of shrinks) or properly diagnosed. It’s that being psychiatrically labelled has caused damage to a lot of us. They are simply descriptions of behaviour and the behaviour can be stated as is, without resorting to circular, stigmatising, truth-removing, and easily abusable psychiatric labels to people sentencing them to a life of fear, discrimination, unwarrantedly nasty behaviour from others, undeserved stigma and also biologising their life (because once labelled, the narrative of bad genes and bad brains starts).

          MadinAmerica is one of the few places on the internet which prevents individuals who do those nasty things to us from doing those things. Because there are innumerable places on the internet which attract people who WANT to do those things and can’t resist the temptation to. Sometimes they end up here too and gladly, they are swiftly stopped.

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      • Technically, both the “heavy cannabis use” and the “antipsychotic use” are causing cognitive damage. Both individually cause cognitive damage. It may be the case that the “heavy cannabis use” is a coping mechanism for the “antipsychotic use”. That makes sense.

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        • “It may be the case that the “heavy cannabis use” is a coping mechanism for the “antipsychotic use”. That makes sense.”

          Only half makes sense. The other half is:

          It may be the case that the antipsychotic use is a coping mechanism for the heavy antipsychotic use.


          There likely is a small number — a very small number — of people that use one drug to counter the effects of the other drug and that stopping one drug would result in a dependence on two drugs being resolved.

          Yet as that stands true for a very tiny number of people this observation is useless for most of the people affected, most of the time.

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          • “There likely is a small number — a very small number — of people that use one drug to counter the effects of the other drug and that stopping one drug would result in a dependence on two drugs being resolved.”

            Not a small number at all. That’s what polypharmacy is all about. SSRIs cause mania? Add a “mood-stabiliser”. SSRIs cause sexual dysfunction? Add bupripion. Some drug causes tremors? Add an anti-tremor drug. That’s one of the ways in which people end up on drug combos. It is common in psychiatry.

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  2. Cannabis’ effects on brains varies from person to person. I tried it maybe 6 to 12 times but wish I never had. Others I know have been adversely affected by it as well but, equally, I know plenty of people who show no signs of problems despite decades of use.

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  3. I’ve seen people basically live off these drugs and have no unwanted effects. For many they do cause unwanted effects.
    Psychiatric drugs however cause for the majority, unwanted effects, and no solutions to issues.
    There is much more harm from psych drugs than any other drug but the difference is that there are actually prescriptions for the harmful ones.

    What to do in the case of unwanted feelings or experiences remains the question. And so far, it should only remain an open ended question with attempts to correct an ill functioning human race. Attempts that start where it makes most sense.

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    • “I’ve seen people basically live off these drugs and have no unwanted effects. For many they do cause unwanted effects.”

      A big reason for this is because cannabis requires experimentation to figure out what proportion of THC and CBD works for the individual. When one has access to legal and, importantly, chemically tested forms of the drug, one is able to determine which ratio of cannabinoids produces the desired effect for the individual.

      I *always* reject untested (read: illicit) supplies of cannabis because I already know that higher percentages of THC produce undesirable effects. I’d rather go without pain relief and calming than experience the paranoia and depression that higher amounts of THC result in for me. But everyone is different and I personally know people who do not get sufficient pain relief from the same chemical makeup (strains) that work for me. Those people require higher amounts of THC and it does not result in “psychosis”. And this is why talking about “cannabis” as if all cannabis is the same thing is wrong and uneducated.

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        • Everybody Lies wrote: “What makes you say coming off street drugs is easier than pharmaceuticals. You’re also saying that coming off psychiatric drugs is disastrous. Better to stay on them then? Oh, the anxiety, it’s just too much. Poor me.”

          The drug most people suffer from and consistently fail to come off is processed non-nutritious sugar. It’s normalised and all-pervasive and very difficult to avoid but devastatingly dependence forming.

          More than any other drug it kills, induces diseases, fucks up the brain, all the major organs, the moods, sleep, the blood, trains people to be impulsive and instant-gratification morons.

          But putting aside the many drugs we do not culturally accept as drugs despite the fact they are drugs…

          It is arguably easier to come off illicit drugs over and above licit drugs because there is massive societal stigmatisation of illicit drugs and large investment programmes to help people noyt just come off them but to transform their lives and I for one endorse and condone such use of resources.

          If you want to for instance come off SSRIs you will to this day have a struggle to even convince people you’re hooked when in fact you are indeed hooked.

          Imagine if someone trying to come off heroin was met with the same incredulity as someone wanting to get off SSRI.

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  4. Schizotypal personality disorder is an euphemism for “this persons personality isn’t normal enough and as a result they get stigmatized and abused therefore the problem is them” This study design doesn’t tell us marijuana increase this type of personality or if this type of personality uses more marijuana. Even if marijuana increases this type of personality (which this study does not scientifically show) it’s not a bad thing minus societies habit of hurting people who are different.

    I don’t have access to the exact “cognitive” tests and results of the study so it is impossible to tell if the subjectively stated cognitive difference is actually a real negative. A test can claim someone is mentally defective simply because they fail to agree with the majority. The psychiatric PANSS test says someone is psychotic if they simply disagree with psychiatry.

    Marijuana is a drug that has negative effects. Smoking is not healthy period. The problem I have is claiming things are bad because they don’t advance a bigoted view of how people should be. This study does this and therefore nothing it says can be taken as honest or accurate. Ironically this study hurts the cause of reducing marijuana use. Stating the negative effects from marijuana is that non users are bigoted against it and the drug causes a form of rebellion results in more marijuana users than if other negative effects are portrayed.

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    • “Marijuana is a drug that has negative effects.”

      Marijuana is a holdover term from the drug war. The plant is called cannabis. Cannabis CAN have negative effects. It can also have very positive effects, including helping those living with chronic pain to withdraw from long term opioid use, present company included. I was prescribed opioid pain relievers for over ten years. The resulting chronic constipation led to my prolapse and hysterectomy. This was a major surgery that had long term medical complications Including two follow up corrective surgeries. Using cannabis would have been (and has been) much better for me medically.

      I completely agree with you on smoking, though. There are better ways to ingest the drug that aren’t harmful to the lung tissues.

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      • Everybody Lies: “Smoking is not healthy period? It is, if you are taking antipsychotics. It reduces the negative impact of neuroleptics by up to 50%. It’s a survival mechanism.”

        Well that’s the finding of a poorly designed study that gives science a bad name.

        What is actually being discovered?

        That stimulating the brain’s nicotinergic system clearly affects the brain’s dopaminergic system?

        Or that both neuroleptic drugs and nicotine-drugs affect multiple brain systems.

        How much sugar were the study particpants consuming? Aspartame? Environmental pollutants?

        Generally there are no reliable ways to control these variables without quite massive studies involving millions at the very least.

        So let’s just play pretend with 75 people who we’ll cherry pick with one beady eye on the desired outcome…

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  5. “ Limited longitudinal testing was done twice, 6 years apart, which confirmed previous findings, but only 4 participants were involved in these later tests.”

    Micah, this is not science. FOUR participants? This is propaganda.

    “Cannabis” is not a homogenous substance and research cannot identify “cannabis” as the subject. Research that does not control for cannabinoids levels is useless. The legal definition defining hemp from cannabis is that any plant with less than 1% THC is hemp and over that threshold is cannabis. I guarantee you that cannabis with less than 8-10% THC does NOT produce these effects despite producing a pleasant feeling for the user. The vast majority of cannabis products contain 20-30% and even higher amounts of THC. Additionally, strains with an equal or greater proportion of CBD to THC also will NOT produce these effects, even with relatively large amounts of THC as CBD modulates the effects of THC.

    It is extremely bad science and reporting to refer to cannabis in a generic sense without clearly indicating the dosage of various cannabinoids participants were exposed to.

    Thirdly, there is an implicit assumption here that western values regarding sanity and “psychosis” are both applicable and preferable to other cultures values. This study was produced in studying another culture but has taken the cannabis use and isolated it from all of the other aspects of that culture. It presumes that there is no benefit to “psychosis” in other cultural contexts such as identifying and training potential shamans. It reads like it’s straight from the European colonizer’s handbook.

    This report and the original “study” should both be retracted.

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  6. Of course, correlation does not equal causation.
    Maybe read the book Schizophrenia: The Sacred Symbol of Psychiatry by psychiatrist Thomas Szasz.
    Psychosis can only be diagnosed through talking with someone and observing their behaviors.
    According to psychiatrist Thomas Szasz, attempting to find an organic etiology of any mental disorder is like trying to measure the caloric content of food for thought. Once an objective organic etiology is found for a psychiatric disorder, then what one has is a neurological disorder that happens to show evidence through language and behavior, it seems.
    Some of the studies you mentioned might have racial bias and their results might be partially due to institutionalized racism and/or implicit bias, maybe.
    Bipoc are diagnosed with psychosis more than whites.
    On the following YouTube link, this gentleman is an LMFT and created a YouTube video showing how some psychiatric marketing quizzes can sometimes lure potential “patients” through how they are worded, and because of how individuals use language in their minds. https://www.youtube.com/watch?v=Jh6LBWAdYok
    The history of neurosyphilis might be relevant too. Once a mental disorder is proven to be a brain disorder, psychiatrists should no longer treat the disorder, neurologists should. If psychiatric disorders are actually brain disorders then neurologists should treat them.
    I could write more about this right now, however, I will leave this as is for now. Cheers and serenity. Michael Ten, MA

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      • Science now means whatever those with authority say it is. At least that is how psychiatry and the public view it.

        If psychiatry says drugs that increase serotonin and dopamine cure depression because of a chemical imbalance it is science. When the person is still depressed and psychiatrists say they need to add a drug that blocks dopamine and serotonin to cure a chemical imbalance it is science. It may seem to contradict itself but that is because everyone who is not a loyal psychiatrist lacks insight.

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          • Which is an important message to get out to young people who think they are interested in “science”. Many are interested in theories, and don’t realize that theories are not science.
            But theories will give you a job, in the theory field, until someone comes up with a better theory.
            It should be referred to as the “world of theories”

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    • Neurosyphilis has similar effects as neuro Lyme, by the way, in the way the infection effects the central nervous system and brain. But I don’t agree that neurologists should be treating it as neither condition is strictly a neurological disorder but instead an infection that is treatable. It is rather in the domain of infectious diseases specialists.

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  7. I have no doubt marijuana can be harmful, having experienced a feeling of being split into two personalities after smoking pot when I was in my 20s. The feeling lasted for several days. It wasn’t the pot, because no one else had a similar experience and it wasn’t an underlying psychosis because it never happened again. Nonetheless, the study cited here is not convincing and seems to have been poorly done.

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