Cannabis and Violence


Most researchers agree that the effect of cannabis is normally (i.e. in the case of the average abuser) to calm and to induce passivity rather than to stimulate aggressiveness. However, owing to the dramatic nature of the effects produced by cannabis on the mind and to clinical observations of violent acts, the question of whether cannabis might be associated with aggressiveness and acts of violence has been raised repeatedly. A large number of commissions and conferences have focused on this issue (). In all cases, the conclusion reached has been a similar exonerative verdict of “not guilty”.

Abel () does, however, point out that a weakness shared by all these commission reports is that in general, they have not looked at the question of the effect produced by cannabis on individuals who are especially vulnerable in this respect – above all, mentally fragile people with low levels of impulse control and people with psychoses, borderline psychotic conditions, profound personality disorders and brain damage.

There seems to be no study that shows, in a methodologically satisfactory manner, that cannabis is in any way linked with violence. The explanation may have something to do with Abel’s above-mentioned point of view. If you add in the fact that the primary objective of most studies is to identify pure effects of cannabis, the findings are easier to understand. In stable, mentally healthy people, the calming effects usually predominate. As can be seen from the pilot studies referred to below, it appears to be precisely the combination with a mental disorder that may sometimes lead to unfortunate consequences.

When it comes to alcohol and illegal drugs other than cannabis, we increasingly speak in terms of “dual diagnosis” or “co-morbidity”, taking an interest in the interaction between two or more conditions. It seems to be time that we studied not only the interaction between cannabis and schizophrenia but also the connection between, for instance, cannabis and various personality disorders.

Moreover, the studies mentioned below remind us once again how important it is to reflect on the interaction between alcohol and cannabis.

A Few Pilot Studies in the Field of Cannabis and Violence

Spunt et al. () conducted a study which shows that in certain cases of aggravated violent crime there is probably a connection with cannabis intoxication. They interviewed 268 people sent to prison for murders committed in New York State during 1984. Of these inmates, 73 had been under the influence of cannabis when committing their murder; and of these, 18 were of the opinion that there was a link between the murder and the effects of cannabis. The persons interviewed were also asked how marijuana smoking affected them. Four of them gave answers along the lines of “it made me aggressive, violent”, one answered “when I am high I just lose control …”, and another said “I don’t think I had done anything if I hadn’t been under the influence”. Four interviewees gave answers of the type “it lowered my inhibitions”, and two replied along the lines of “it made me feel paranoid”.

Fifteen of the eighteen murderers who had been under the influence of cannabis were also under the influence of alcohol or an illegal drug other than cannabis at the time of the murder. Nine of these said that they thought the combination of cannabis with alcohol – or of cannabis with another drug – was an important factor in their committing the crime. One of the three who were under the influence of only cannabis and alcohol explained the effect in the following manner: “One alone you can handle – but two together confuse your mind.” Another of them said: “The alcohol took away my inhibition and the pot made me crazy.” And the third of them observed: “The combined effect made me lose self-control.”

Niveau and Dang () have accounted for twelve cases of aggravated violent crime, all committed in Geneva in the period 1996–2000. Initially, there was a much larger study group, but those with poly-drug abuse were excluded. When committing the crime, the individuals in question were under the influence of cannabis only. Of the twelve subjects, five had a previously known personality disorder and three had other psychiatric disorders.

At the time of the crime, all twelve were suffering from severe negative effects caused by cannabis consumption: four of them experienced acute psychotic conditions and one suffered a relapse into or an exacerbation of chronic paranoid psychosis. A further three of them experienced negative reactions such as intensive anxiety (the description is somewhat unclear on this point) and three were affected by delirium. One patient had a “mood disorder”.

The report is densely written and my space here is limited. What I wish to emphasise most of all is what appears to be, at least in some cases, an unfortunate combination of (mainly psychotic) vulnerability, cannabis and a stressful situation whose joint effect has been to provoke psychosis, delirium and attacks of rage, all with a strong element of aggression. It should be noted that some of the processes described here are given scientific explanations in posts. The account of the study by van Os et al. shows how great the risk of psychosis outbreak is in vulnerable people who have a history of psychiatric problems, especially in the psychotic direction.

These authors and others maintain that, given these insights, cannabis abuse must be taken more seriously.

In other words, here we see two shifts in perspective as regards the dangerousness of cannabis. There is growing interest in dual diagnoses involving cannabis abuse as one of the disorders, and there is also growing interest in the interaction between alcohol and cannabis.


Selections from the book: “Adverse Health Consequences of Cannabis Use. A Survey of Scientific Studies Published up to and including the Autumn of 2003