What is the relationship between cannabis use and violent behaviour?

Schoeler T, Theobald D, Pingualt J-B, Farrington DP, Jennings WG, Piquero AR, Coid JW, Bhattacharyya S. Continuity of cannabis use and violent offending over the life course. Psychological Medicine 2016;46:1663-1677 (abstract and link to full article)

Why was this study conducted?

Previous research suggests violent behaviour or delinquency and aggression in adolescence may be associated with cannabis use. To study this association, researchers analyzed data from a prospective cohort study of 411 mostly Caucasian boys born around 1953 and living in a homogeneous, working-class urban part of London. Outcomes for the study included a violent crime conviction (VC), defined as conviction for robbery, assault, threatening behaviour, or possessing an offensive weapon, and self-reported violent incidents (SR-V) involving assaults, fights or use of a weapon. The predictor variables were ever cannabis use at any of three assessment periods (ages 18, 32 and 48 years) and continuity of cannabis use (never, at one assessment, at two assessments, or at all three assessments). A number of possible factors and childhood risk factors were also modelled in the analysis.

What does this study add?

In multivariate logistic regression, compared to never-users, there was no significantly higher risk of VC or SR-V for boys who reported cannabis use at one or two time points. However, those that reported cannabis use at all three assessments had significantly increased risks of VC (OR=7.08, 95% CI 2.19-23.59) and SR-V (OR=8.94, 95% CI 2.37-46.21). This effect was greater than that associated with having an antisocial personality disorder (for VC OR=3.43, 95% CI 1.59-7.52 and for SR-V OR=2.15, 95% CI 1.19-3.91). Other factors, such as a family history of crime or low social class were associated with significant increased risks of VC but not SR-V.

Is there anything else I should know?

This study suggests that persistent cannabis use from adolescence to adulthood may be associated with violent behaviour. The authors point out that as the study only included males, so the results cannot be generalized to females. In addition, this study is limited to a single, mostly homogeneous setting and culture. Finally, the authors could not study the effect of the frequency of use or type of cannabis, which some researchers have found to moderate the effects of cannabis on violence.