Cannabis consumption associated with increased violent behavior in young people in psychiatric care

The association between persistent cannabis use and violence is stronger than that associated with alcohol or cocaine, concludes a study in Frontiers in Psychiatry.


Users who reported at each follow-up visit that they continued to smoke cannabis presented an increased risk (+144%) of violent behavior. Image: Shutterstock.

The association between persistent cannabis use and violence is stronger than that associated with alcohol or cocaine.

— University of Montreal

A new study by researchers at the Institut en santé mentale de Montréal demonstrates that sustained used of cannabis is associated with an increase in violent behavior in young people after discharge from a psychiatric hospital.

The research by Dr. Alexandre Dumais (MD, PhD, FRCPC, psychiatrist at the Institut Philippe Pinel) and Dr. Stéphane Potvin (PhD, professor at the Université de Montréal), which studied 1,136 patients (from 18 to 40 years of age) with mental illnesses who had been seen five times during the year after discharge, took into account substance use and the onset of violent behavior.

Previous research has already shown that a cannabis use disorder is associated with violent behavior. According to this new study published in Frontiers in Psychiatry, users who reported at each follow-up visit that they continued to smoke cannabis presented an increased risk (+144%) of violent behavior.

These results also confirm the detrimental role of chronic cannabis use in patients with mental illness. According to the principal researcher Alexandre Dumais (MD, PhD, FRCPC): “an interesting feature of our results is that the association between persistent cannabis use and violence is stronger than that associated with alcohol or cocaine.”

Indicator for external follow-up

Persistent cannabis use should therefore be considered as an indicator of future violent behavior in patients who leave a psychiatric hospital for follow-up in an outpatient clinic, although the researcher points out that this behavior tends to fade with time.

“This decrease could be explained by better adherence to treatment (the patient becomes more involved in their treatment over time) and by better support from their entourage. Even though we observed that violent behaviour tended to decrease during follow-up periods, the association remained statistically significant,” noted Dr. Dumais.

The research results also suggest that there is no reciprocal relationship, that is, the use of cannabis resulted in future violent behavior and not the reverse (for example, a violent person might use cannabis following an episode of violent behavior to reduce their tension), as was suggested by previous studies.

The effects of cannabis on the brain

A recent meta-analysis of neuroimaging studies demonstrated that chronic cannabis users have deficits in the prefrontal cortex, a part of the brain that inhibits impulsive behavior.

These results are important because they offer additional information to young adults, who can evaluate the risks of cannabis before deciding whether or not to use it. They will also serve as a tool to develop strategies to prevent the risk of violence associated with cannabis, since these risks have important consequences, both socially and for the health of young adults and for society in general.

Original research article: Persistency of Cannabis Use Predicts Violence following Acute Psychiatric Discharge

Corresponding author: Alexandre Dumais

This study was funded by the Fonds de la recherche du Quebec-Santé.

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17 Comments on Cannabis consumption associated with increased violent behavior in young people in psychiatric care

  1. The headline is completely misleading, as it impies causality. Fortunately, the second line of the article states results clearly, talking about associations.

  2. Joao R. L. Menezes // November 17, 2017 at 1:15 pm // Reply

    What recent meta-analysis showed deficits in the prefrontal brain?

  3. Does the increase in violent behavior include attacks against psychiatric hospital psychiatrists?

  4. Concerned Scientist // November 17, 2017 at 2:27 pm // Reply

    How can you post a headline that implies causation? That’s extremely misleading and unscientific.

    Correlation is not equal to causation.

  5. The problem with this study is since, these are psychiatric patients they are probably already taking other mood altering medications. Is it the psychiatric medicines causing the violence, is it a combination of marijuana and psychiatric medication causing the violence. Very difficult to correlate when some many variables are involved

  6. Observing cannabis use over the past 45 years with hundreds of people, this article and the opinions are definitely incorrect.

  7. Is it not conceivable that cannabis is a form of self-medication for these patients, to alleviate their pre-existent violent tendencies? (In other words, violent behavior may have been even more prevalent if they had not smoked cannabis.) If I remember correctly, a similar connection has been suggested for schizophrenia and nicotine (based on the high proportion of smokers in these patients).

  8. The short headline, prior to clicking, is still causal language. Please represent science accurately.
    Secondly, I find these results quite surprising, and i will read the article to examine if they statistically controlled for potential confounding variables, like history of personal maltreatment, history of other drug use, etc.

  9. This is an embarrassment to science.

  10. This was a study of self-reports. The authors discuss this as a major limitation (as they should have). This kind of work does not include empirical support for statements such as “These results also confirm the detrimental role of chronic cannabis use in patients with mental illness.”

    This news piece is still very misleading – it does not accurately interpret the actual paper.

  11. The comments here are divorced from personal experience. Anyone who has observed cannabis in action can see its personality changes, schizophrenic and slight effects toward violent reactivity. Having said that, it still the best recreational and self medication substance out there, unless it becomes a psychological addiction, which it will in tye majority of cases, and during these highly stressful times. It should be decriminalized for that reason alone. The alternative is opiate addiction, which is certainly tragic.

  12. Nico Kolokythas // November 17, 2017 at 5:06 pm // Reply

    In the original paper

    In Limitations
    While a large longitudinal study design was employed to assess the association between cannabis use and violence amid psychiatric patients, a few limitations are worth to bear in mind. First, the sample size decreased in statistical analyses due to individuals lost across follow-ups and missing data. However, this does not take away the importance of our findings, considering the large sample size remaining. Second, since our hypotheses were not based on the effect of cannabis use on the number of violent acts, we focused solely on the presence of a violent behavior during follow-up periods.

    First sentence talks about associations and then the associations become effects. Shouldn’t the writers be a little bit more clear? It’s an observational study, therefore, effects cannot be claimed.

    From the original paper:
    “cannabis use over time was a significant predictor of violence when consumed at two time-points (OR = 1.71, 95% CI: 1.08–2.70, p < 0.05), three time-points (OR = 2.08, 95% CI: 1.16–3.74, p < 0.05), and four time-points (OR = 2.44, 95% CI: 1.06–5.63, p < 0.05)."
    "Moreover, only the continuous use of alcohol over the four follow-up periods significantly increased the odds of future violent behaviors (OR = 2.32, 95% CI: 1.25–4.28, p < 0.001)."

    But the focus is on cannabis, is there a confirmation bias hiding somewhere there?!

    • a) If you’re going to think I’m crazy anyways, then why not act crazy? This could be habit forming in a sense and explain the results. If you’ve ever been a teenager or met a teenager, you may be able to understand that this is a pretty reasonable claim. Sure, you can try to patronize me or humiliate me in numerous ways to try to force the result of abandoning “crazy” behaviour, but in teenagers this could easily cause lashing out, depression, or other such things.

      b) The fact of being illegal puts the trade in criminal hands, which then increases the probability of trade-related violence.

      c) If you i) believe the result and ii) apply it in biased manner in real world decisions (e.g., to assume that the causal relationship would apply to every single case, as opposed to, say, the 1% of cases identified as different), then your own natural mental defects related to processing bias, etc., will at the very least cause the result to be exaggerated, because the pot smoker is more likely to get a record for the same act that another person wouldn’t.

      I personally believe that cannabis causally operates in the other direction (less, not more, violence prone). All of the above explanations show how that can be true even if the statistical results are factually correct with respect to i) considering the subset who’d received a diagnostic opinion of mental illness followed by ii) the differential of recorded incidents of physical altercation between those who’d smoked pot before and not, in particular due to the effects of b) and c).

      If you cannot acknowledge the direction of effect, if not necessarily the relative magnitude, of b) and c) — not necessarliy in a paper, although I think you should, but I mean internally, in your head, understand that this is OBVIOUSLY the case — then you shouldn’t be within 1000 miles of telling people who’s defective and/or why.

  13. Clickbait headlines in emails *help no one* (I’m glad the headline on the article is changed. Also please provide references to referred content, especially that “recent meta-analysis of neuroimaging studies” showing “chronic cannabis users have deficits in the prefrontal cortex”– to say that and not allow folks to evaluate the evidence is unhelpful too

  14. It is not mentioned, but in my experience, they must be talking about the sativa type. The poor conclussion in this study is like pretending to put all the mood stabilizers in the same balance.

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