Cannabis and Violence: Examining the Complex Relationship

The association between cannabis use and violent behaviour has been a subject of intense debate and research in recent years. As cannabis legalisation spreads across many Western countries, understanding this relationship becomes increasingly important for public health and policy. This article explores the current evidence on the link between cannabis use and violence, particularly among adolescents and young adults.

Multiple studies have found a moderate association between cannabis use and an increased risk of perpetrating physical violence. A meta-analysis of 30 studies, encompassing nearly 300,000 participants, found that cannabis users had approximately twice the odds of engaging in violent behaviour compared to non-users. This association remained significant even after controlling for various confounding factors.

Longitudinal Evidence

One of the most compelling pieces of evidence comes from the Cambridge Study of Delinquent Development, a longitudinal study following 411 young males from age 8 to 56. This research found that continued cannabis use (at ages 18, 32, and 48) was associated with a significantly higher risk of subsequent violent behaviour. Compared to never-users, persistent cannabis users had:

7.1 times higher odds of violent convictions
8.9 times higher odds of self-reported violent behaviour

Importantly, this effect persisted after controlling for other potential risk factors for violence. The study employed multiple analytical approaches, including fixed-effects analysis and cross-lagged modelling, which suggest that the relationship between cannabis use and violence cannot be fully explained by other unobserved time-invariant factors.

Bi-Directional Relationship

Key findings from the Cambridge Study of Delinquent Development include:

Cannabis use predicting violence: Continued exposure to cannabis (use at ages 18, 32, and 48) was associated with a significantly higher risk of subsequent violent behaviour. This was true for both officially recorded violent convictions (OR 7.1, 95% CI 2.19-23.59) and self-reported violence (OR 8.9, 95% CI 2.37-46.21).
Persistence of effect: The association between cannabis use and violence remained significant even after controlling for other potential risk factors for violence.
Ruling out time-invariant factors: Fixed-effects analysis and cross-lagged modelling indicated that the relationship between cannabis use and violence could not be explained by other unobserved time-invariant factors.
Bi-directional relationship: Importantly, these analyses uncovered evidence for a bi-directional relationship. This means that while cannabis use predicted future violent behaviour, engaging in violence also increased the likelihood of subsequent cannabis use.

This bi-directional relationship highlights the complex interplay between substance use and violent behaviour. It suggests that interventions aimed at reducing either cannabis use or violence may have positive effects on both outcomes. For example, programs targeting violence reduction might also lead to decreased cannabis use, and vice versa.

Dose-Response Relationship

In addition to the bi-directional nature of the cannabis-violence association, evidence suggests a dose-response relationship, where heavier or more persistent cannabis use is associated with a greater risk of violent behaviour.

A meta-analysis of 30 studies, encompassing 296,815 adolescents and young adults, found:

Overall association: A moderate association between cannabis use and the perpetration of physical violence (pooled OR 2.11, 95% CI 1.64-2.72).
Dose-response effect: The risk of violence was higher for persistent heavy users (OR 2.81, 95% CI 1.68-4.74) compared to past-year users (OR 2.05, 95% CI 1.5-2.8) and lifetime users (OR 1.94, 95% CI 1.29-2.93).
Consistency across study designs: This dose-response relationship was observed in both cross-sectional and longitudinal studies, and persisted even after adjusting for potential confounding factors.

The dose-response relationship is further supported by other studies:

Schoeler et al. found that the adverse effects of cannabis use on subsequent violent behaviour throughout the lifespan were driven by continued use, suggesting a dose-response relationship.
In populations with psychosis, where cannabis use is particularly prevalent, higher rates of violent behaviour are observed among cannabis users. For instance, in subgroups of violent patients with psychosis, cannabis use rates range from 40-49% in chronic patients and 44-64% in those in the early phase of psychosis.

Potential Mechanisms

Several mechanisms have been proposed to explain the link between cannabis use and violence:

Physiological and psychological effects: Cannabis may impair the ability to control aggressive impulses, induce feelings of paranoia and anxiety, and increase physiological arousal.
Withdrawal symptoms: In dependent users, cannabis withdrawal can lead to irritability and emotional outbursts.
Neurobiological changes: Chronic cannabis use, especially when initiated at a young age, may disrupt normal brain maturation processes. This could affect neural structures associated with inhibition and sensation-seeking, potentially increasing the risk of violent behaviour.
Social and contextual factors: Cannabis users may have a higher tolerance for delinquency, engage in more risk-taking behaviours, and be more likely to use other substances associated with violence.

Cannabis Use in Early Psychosis

The relationship between cannabis use and violence becomes even more pronounced in individuals experiencing early-phase psychosis (EPP). Studies have shown high rates of cannabis use (29-38%) among young people with psychotic disorders. In subgroups of violent patients with psychosis, cannabis use rates are even higher:

40-49% in chronic patients
44-64% in early-phase psychosis
Up to 70% in prisoners at high risk for developing psychosis

These findings suggest that cannabis use may play a particularly significant role in violent behaviour among individuals with or at risk for psychotic disorders.

Cannabis Use in Psychiatric Patients

Recent research has shed light on the relationship between cannabis use and violence among psychiatric patients, a population where cannabis use is twice as prevalent compared to the general population. A prospective study using data from the MacArthur Risk Assessment Study, which followed 1,136 recently discharged psychiatric patients, found a unidirectional association between cannabis use and violence.

Key findings from this study include:

Continuity of cannabis use across multiple time waves was associated with increased risks of future violent behaviour.
Patients who reported using cannabis at each follow-up period were 2.44 times more likely to display violent behaviours (OR = 2.44, 95% CI: 1.06–5.63, p < 0.05).
The longer individuals reported using cannabis after a psychiatric discharge, the more likely they were to be violent in subsequent time periods.

These results are particularly relevant for clinical practice and violence risk management in psychiatric settings. They suggest that addressing ongoing cannabis use among psychiatric patients could be an important factor in reducing the risk of violent behaviour.

General Population Studies

Recent research has also strengthened the evidence for a link between cannabis use and violence in the general population:

A pioneering study found that regular cannabis users run an increased risk of using violence against others. The relationship between cannabis and violence was found to be more constant than the relationship between alcohol or cocaine use and violence.
A landmark study of almost 300,000 teenagers and young adults found that people who regularly smoke cannabis are almost three times more likely to commit a violent offence as those who abstain from the drug.
Scientists involved in this study believe that over time, prolonged cannabis use profoundly alters the brain, making the user less able to control their temper.

These findings from large-scale studies add weight to the growing body of evidence linking cannabis use to an increased risk of violent behaviour.

Limitations and Future Directions

While the evidence for an association between cannabis use and violence is growing, several limitations should be noted:

Heterogeneity in study methods and definitions of violence
Insufficient control for important confounding factors in some studies
Limited assessment of cannabis exposure patterns (e.g., potency, frequency)
Need for more longitudinal studies with large sample sizes

Future research should focus on clarifying the dose-response relationship, investigating the impact of different cannabis strains and potencies, and exploring the interaction between cannabis use and other risk factors for violence.

The accumulating evidence from both general population studies and research on psychiatric patients strongly suggests a relationship between cannabis use and an increased risk of violent behaviour. This association appears to be particularly pronounced with persistent, long-term use. As cannabis legalisation continues to expand, these findings highlight the importance of educating the public, especially young people and those with mental health conditions, about the potential risks associated with cannabis use, including the risk of future violence.

The implications of these findings extend beyond public health to clinical practice, especially in psychiatric settings where cannabis use is more prevalent. Addressing cannabis use could be an important component of violence risk management strategies for psychiatric patients.