With the growing number of US states and countries that have liberalized their cannabis laws by decriminalizing or legalizing its use, it has become important to assess the potential harmful effects of cannabis use. Youths are particularly vulnerable to the harmful consequences of cannabis use with effects that can continue into adulthood. Interest in the effects of cannabis has so far mainly focused on the health of the users themselves. However, harm to others, such as violent behavior, has been less considered. Moreover, the association between cannabis use and violence among youths continues to be debated without any consensus having emerged from scientific literature.

Exploring the Cannabis–Violence Association

We conducted a meta-analysis to assess the extent to which cannabis use was associated with interpersonal physical violence in youths, and had our results published in the American Journal of Psychiatry. Several electronic databases were searched from their inception date until July 2019 to find publications assessing both cannabis use and the perpetration of physical violence in a sample of young people under the age of 30. After screening 11,348 potential studies, 30 study arms were included in our review leading to a total sample of nearly 300,000 adolescents and young adults.

The results showed that young cannabis users were approximately twice as likely to perpetrate acts of physical violence as were non-users (Table). These results followed three other meta-analyses on the cannabis-violence association concerning dating violence and individuals with severe psychiatric disorders, which also showed an overall moderate association. More specifically, we also examined the association for different types of cannabis use patterns (eg, lifetime, occasional, frequent use) to verify the existence of a “dose-response” relationship. Our preliminary findings suggested a potential dose-response relationship; that is, more frequent cannabis users were at increased risk of perpetrating violence. Finally, we were interested in the direction of the association—whether the use of cannabis would lead to future violence, or if externalized behaviors, such as violence, would lead to the initiation of cannabis use. Our findings from longitudinal studies supported our hypothesis that cannabis use during adolescence may lead individuals to later perpetrate physical violence in early adulthood.

Beyond the Results

Several research questions still need to be further investigated. First, studies should continue to explore the direction of the association, as there is still little literature in youth populations. This is particularly important as some literature has also pointed toward a possible reverse relationship: physical violence in adolescents and young adults may increase the risk of initiating the use of cannabis later in life. Second, studies should further analyze and disentangle different cannabis exposure patterns (eg, type of cannabis, frequency, dosage, potency) in aims of better understanding how cannabis may be associated with violence. Such analyses may also aid to determine a level at which detrimental effects of cannabis use occur. Lastly, studies should attempt to better define the specific mechanism linking cannabis use and violence in youths, as several potential mechanisms have been put forth. For instance, some authors have stated that violence may be due to the psychophysiological effects of cannabis use (eg, increased aggression while intoxicated or during withdrawal), whereas others have noted that it may be due to the interaction between users and their social environments (eg, increased aggression associated to drug use in the black market).

In the meantime, our research shows that cannabis in youths is a risk factor for violence, which should be considered by professionals and requires effective strategies to limit its negative effects. Clinicians should screen their patients for cannabis use patterns and related adverse effects of violence. Psychoeducation on cannabis use should particularly be encouraged. Hence, clinicians should offer patients educational information for decision-making and dissuade them from commencing and embracing more chronic patterns of use that may increase the risk of negative outcomes, such as violence. Moreover, for those who show signs of problematic use, they should be directed towards resources and treatments (eg, cognitive behavioral therapy) that may help to reduce their use of cannabis.