Dr Des Corrigan examines whether there is a proven link between cannabis use and violent behaviour
A trial at the end of July, where a man was found guilty of murdering his wife, centred around the possible role of cannabis use as a contributing factor. This case ties-in with a report earlier this year in The Irish Times, drawing attention to a link between cannabis use and four murders that had been dealt with by Irish courts. Many readers may have been surprised by this because the perception of cannabis users has often been that they can be passive, lethargic and apathetic, particularly if they are daily users and unlike far too many alcohol users, they would not be seen as aggressive or violent individuals.
This could be an excessively naïve view, not least because the Diagnostic and Statistical Manual of Mental Disorders (DSM-5) lists irritability, anger or aggression as one of seven behavioural or emotional symptoms of withdrawal from cannabis that are used in the diagnosis of Cannabis Use Disorder (CUD). One study reported in a 2019 paper from Drug and Alcohol Dependence that of those who used cannabis more than three times a week over a period of a year, 72 per cent recorded those negative hostile feelings after stopping cannabis use. So, it is feasible that withdrawal could prompt violent behaviour, but the literature suggests additional possibilities.
A 2020 review in Frontiers in Psychiatry refers to violence and cannabis use as a “forgotten aspect in the era of liberalising cannabis”. It points out that there is meta-analytical evidence pointing towards an association between cannabis use and violence, and that this link is stronger in the case of individuals with severe mental disorders. It also highlights a potential dose relationship, with larger effects seen in more frequent users. On that basis, it is also reasonable to speculate that the use of the more potent forms that now dominate the market — such as concentrates and edibles (jellies), with their higher levels of THC and probably also those cannabis products laced with synthetic cannabinoids, as recently outlined by the EU’s Drugs Agency (EUDA) — would add to the risks.
That violence could involve young people, but it could also involve what the review calls “intimate partner aggression perpetration”, which I think most normal people would regard as domestic violence without the need to use four words when two will do. Of the four murders listed in The Irish Times article, two involved domestic violence, while two involved attacks on strangers. All four involved cannabis-induced psychosis on the basis of evidence presented in court. The Mayo case that concluded at the end of July was also a domestic violence case.
While psychosis with its associated paranoia is an obvious explanation for cannabis-induced violence, it is clear from the literature that it is not the only factor at play. A 2023 paper in JAMA Open Network looked at data from over 68,000 adolescents in a national survey. Those who had used cannabis in the past year were more likely to have been in a serious fight and nearly twice as likely to have attacked someone with intent to harm compared to non-users. For those with a diagnosis of CUD, they were over three times more likely to report being in a fight and over four times more likely to have attacked someone.
While the 2020 review mentioned earlier talked about violence being a forgotten issue, it has attracted more attention recently. A paper in the International Journal of Drug Policy earlier this year investigated associations between cannabis use, its frequency, and presence of CUD with violent behaviour among young adults in the USA. It focused on data from 18-to-34 year-olds enrolled in the US National Survey on Drug Use and Health between 2015 and 2019. It found that violent behaviour was highest among males who used cannabis daily but who did not have CUD, but also among those who did have CUD, compared to those who had not used cannabis in the past year. Females who were daily users had higher levels of violent behaviour compared to non-users.
A 2020 meta-analysis in the American Journal of Psychiatry investigated the association between cannabis use and physical violence in young people. Data from a total of 30 studies involving 296,815 young people was analysed. The risk of violence was nearly three times higher in persistent heavy users compared to those who had used cannabis in the past year or who had used at least once in their lifetime. According to the authors, the results demonstrate a moderate but significant association between cannabis use and physical violence, even after adjusting for factors such as other drug use and socio-economic status. They highlight the need to educate young people about this little-known risk and to investigate effective strategies to limit its impact in an era of liberalisation.
According to a 2018 paper in Translational Issues in Psychological Science, titled ‘Marijuana use is associated with intimate partner violence perpetration among men arrested for domestic violence’, there is a small but growing body of research showing that cannabis use is positively and significantly associated with psychological, physical and sexual IPV, as they call it. The authors base this on their study of 269 men arrested for domestic violence in the US and who had been referred by the courts to batterer intervention programmes. Even after controlling for problem alcohol use that is known to be a robust risk factor and variables such as antisocial personality symptoms, there was a statistically significant association.
It is also important not to lose sight of the impact of other drugs, particularly alcohol and psychostimulants
A more recent (2020) study in the American Journal of Addictions looked at the association between cannabis use and IPV among couples with alcohol or drug misuse issues. After accounting for variables such as frequency of alcohol and stimulant use in the 30 couples, it was found that greater IPV physical and psychological victimisation was linked to the quantity and frequency of cannabis use, as confirmed by urine drug screens.
It is difficult to state whether the increased violent behaviour and the often-tragic consequences thereof is due to cannabis use per se, or whether people prone to violence are more likely to use cannabis. It is also important not to lose sight of the impact of other drugs, particularly alcohol and psychostimulants such as cocaine (both powder and crack) and methamphetamine, all of which are well-known to induce extreme violence.
Dr Des Corrigan, Best Contribution in Pharmacy Award (winner), GSK Medical Media Awards 2014, is an Adjunct Associate Professor at the School of Pharmacy and Pharmaceutical Sciences at TCD where he was previously Director and won the Lifetime Achievement Award at the 2009 Pharmacist Awards. He was chair of the Government’s National Advisory Committee on Drugs from 2000 to 2011, having previously chaired the Scientific and Risk Assessment Committees at the EU’s Drugs Agency in Lisbon. He chaired the Advisory Subcommittee on Herbal Medicines and was a member of the Advisory Committee on Human Medicines at the HPRA from 2007 to 2024. He has been a National Expert on Committee 13B (Phytochemistry) at the European Pharmacopoeia in Strasbourg and served on the editorial boards of a number of scientific journals on herbal medicine.
