A recent article in the Wall Street Journal suggests that marijuana is not as safe as it seems. It bases this claim based on two things: first, the author links a rise in violence with the rise in cannabis use; and two: marijuana use can cause psychosis in sensitive individuals, especially people with schizophrenia. The claim is that marijuana will cause schizophrenics to become violent. Essentially, the argument is that marijuana is dangerous because it increases mental illness and those mentally ill patients will become violent. But is there any primary research to back up this claim?
An uncited claim in the article states that "psychosis is a high-risk factor for violence". That claim has a large body of evidence to refute it. Well, does marijuana make these patients violent? A study in the Czech Republic on patients with or without psychosis found a slight increase in violence in psychotic patients, however, they concluded that recent victimization was a better predictor of violent behavior. A much more comprehensive review of the literature revealed that while drug use was an indicator in driving violent behavior, studies examining the misuse of marijuana found no such link. One of the strongest predictors of violent behavior in these patients? Previous criminal history.
There is scant literature examining the links between psychosis, marijuana, and violence in animal studies. Yet there are animal studies that have examined the effect of 9-delta-tetrahydrocannabinol (THC) on aggression. These experiments have found that acute marijuana exposure alone does not make animals more aggressive. Acute and chronic marijuana administration in animals found reductions in both dominance-related and provocation-related aggression in a variety of species, such as rats, mice, fish, squirrel monkeys, and pigeons.
Is there a proposed biological mechanism for how THC (on its own) may decrease violence? One experiment found that genetic knock-out mice for the gene that codes for the CB1 receptor are generally more aggressive than their genetically normal littermates. Another experiment using CB1 knock-outs again found increases aggression. Stimulation of these receptors by THC seems to decrease aggressive behavior acutely and chronically.
This is not to say that marijuana use cannot lead to violence. Yes, marijuana use is usually associated with calmness, euphoria, increased sociability, heightened sensitivity to certain stimuli (e.g., colors), altered perception of time, and increased appetite, but it can also increase impulsivity and psychosis at higher doses. This is extremely important to know if you are going to try marijuana (in a legal state or country, please). If you are vulnerable to mental illness (check your family history), you may want to avoid any substance use. This is not because it will make you psychotically violent, but because you are probably not going to enjoy the experience.
Sources: www.wsj.com, World Psychiatry, Neuropsychiatric Disease and Treatment, PlosOne, Neurotoxicology and Teratology, Aggression and Violent Behavior, psychiatryonline.org, New England Journal of Medicine