The link between marijuana and mental illness is controversial.
The link between marijuana and violence isn’t. Most people don’t even know it exists.
That lack of knowledge contrasts with the popular, and accurate, understanding that alcohol can cause aggression and violence. Anyone who’s ever been in a bar knows that alcohol disinhibits drinkers. For generations, researchers have studied the connection between drinking and auto accidents, domestic abuse, assaults, and even murder.
Marijuana is the world’s most widely used illicit drug, trailing only alcohol as an intoxicant. But despite the historical awareness of the cannabis-violence link, scientists have spent little time examining the connection. Many users simply laugh it off.
The research gap is puzzling. It has probably happened in part because violence research falls between social science and medicine. It is usually run by epidemiologists and public health experts, not doctors. And psychiatrists know far more than nonphysicians about the mental health effects of cannabis.
But the lack of research also reflects the spectrum of psychological effects that marijuana provokes. Some smokers have anxiety and paranoia. Others find themselves hungry, euphoric, and dazed.
When researchers test cannabis’s effects on aggression in laboratory settings, they find the latter effect dominates. In laboratories, most people are less hostile after they’ve smoked, less likely to react to provocations. Those studies have fed the view that cannabis can’t cause violence, a theory that marijuana advocates heavily promote.
In the words of one comedian, Katt Williams, “the side effects—hungry, happy, sleepy. That’s it.”
“The conventional wisdom has been that cannabis doesn’t increase violence because people tend to be mellow when they’re stoned,” said Wayne Hall, an Australian professor at the Centre for Youth Substance Abuse Research at the University of Queensland. Hall has researched drug use and addiction for more than thirty years and advised the World Health Organization on the health effects of cannabis. “It’s an issue that has not been studied that much.”
In fact, a 2008 paper that analyzed earlier research on drug use and crime included only ten studies between 1981 and 2001 which looked at marijuana’s impact. Not one of those specifically focused on violent crime; they mostly looked at offenses such as prostitution and shoplifting.
Yet a surprising number of recent studies have linked marijuana and violence.
Seemingly unnoticed by either academic researchers or the media, the studies have piled up in the last decade. They are usually not focused on cannabis. They are designed to look at other issues, like mental illness among murder defendants in Pittsburgh or bullying in American high schools or even fighting among tourists in Spain. Collectively, they cover tens of thousands of participants and violence that ranges from fighting to firearm use to murder.
Over and over, they have found marijuana use or abuse is strongly associated with violence—more strongly than alcohol, in many cases.
• A 2013 paper in the Journal of Interpersonal Violence used data from a federal survey of more than 12,400 American high school students to examine the link between alcohol, marijuana, and aggression. The researchers’ initial hypothesis, which they published as part of the paper, was that alcohol increased violence while marijuana reduced it.
Instead, they found that students who had recently used marijuana—but not alcohol—were more than three times as likely to be physically aggressive as those who abstained from both, even after adjusting for race and sex. Those who used alcohol, not marijuana, were 2.7 times as likely. (Those who used both were almost 6 times as likely.)
• A 2016 paper in Psychological Medicine examined marijuana use and criminal behavior among 400 boys in London who were followed for more than forty years beginning in 1961; their marijuana use was surveyed when they were 18, 32, and 48. The paper found that marijuana use at all three times was associated with a ninefold increase in violent behavior even after adjusting for other variables.
• A 2013 paper in the American Journal of Psychiatry examined all 278 people charged with homicides, excluding vehicular homicides, in Alleghany County, Pennsylvania, between 2001 and 2005. It found that 90 defendants had been diagnosed with cannabis dependence or abuse, compared to 65 with alcohol dependence or abuse.
• A 2008 paper in the European Journal of Public Health surveyed 3,000 vacationers aged 16 to 35 in the Spanish resorts of Ibiza and Majorca to find out what factors predicted fighting. Cannabis use doubled the risk. Surprisingly, alcohol use did not change it, except for visitors who were drunk more than five days a week, who had a 2.5 times risk for fighting.
• A 2017 paper in Social Psychiatry and Psychiatric Epidemiology surveyed 2,000 young men in Britain and 4,000 in China to see if different factors led to violence in the two countries. Drug abuse was far more common in Britain and associated with a fivefold increase in violence. The study didn’t break out marijuana versus other drugs but noted that “young British men overwhelmingly reported misuse of cannabis.” Alcohol abuse was associated with a threefold increase in violence.
Other studies with similar findings included a 2018 paper on patients entering treatment for substance abuse in Brazil, a 2018 paper on people arrested in Maricopa County, Arizona, a 2017 paper on firearm violence, a 2015 study on veterans with posttraumatic stress disorder, a 2009 paper on emergency room patients in Michigan, a 2004 paper on youth offenders in Pittsburgh, and a 1984 paper on prisoners convicted of homicide in New York State.
Recent papers also show that the link extends to violence in relationships:
• A 2018 study in the journal Translational Issues in Psychological Science showed that among 269 men who had been court-ordered to treatment for domestic violence, marijuana use was associated with physical, psychological, and sexual violence, even after accounting for alcohol use.
• A 2017 analysis of 11 previous studies in Drug and Alcohol Dependence found that marijuana use was associated with a 45 percent increase in violence during dating by adolescents and young adults, compared to a 70 percent increase for alcohol use.
• A 2012 paper in the Journal of Interpersonal Violence examined data from a federal study of 9,421 American teenagers over a 13-year-period. It found that marijuana use was associated with a near-doubling of the risk of committing domestic violence by age 26, even after accounting for factors such as depression and binge drinking. (The study showed that binge drinking was associated with a 31 percent increase in the risk of being a victim of domestic violence but a reduced risk of being a perpetrator.) “We found that any use of marijuana during adolescence and young adulthood increases the risk of intimate partner violence,” the authors wrote. “Consistent users were at greatest risk of perpetration and victimization.”
Other data come from government surveys that examine drug use by prisoners or people who have just been arrested. The most striking data came from a now-discontinued survey by the federal Office of National Drug Control Policy, which for several years ran a project that asked thousands of arrestees to provide urine samples and answer questions about their drug use.
In 2013, the study found that roughly half of arrestees in five big cities screened positive for marijuana—far higher than the rates for other drugs. In Sacramento and Denver, the cities in the survey where medical marijuana was legal at the time, the percentages had trended significantly higher since 2008. About 60 percent of arrestees in Sacramento screened positive.
Unfortunately, the office ended the survey after 2013, so more recent data is not available. But the Australian Institute of Criminology runs a similar survey in that country. In 2013, it found that one-third of detainees had used cannabis less than two days before their arrest; 23 percent said they were dependent on the drug, and almost 9 percent of those attributed their arrest to it (though in some of those cases they had been arrested for possession of it).
The Department of Justice also occasionally surveys prisoners about whether they were using drugs before or during their crimes. Because they rely on self-reported data rather than a urine sample, those surveys probably underreport use. Still, they have similar findings. The most recent study covered the 2007–2009 period and found that 22 percent of prisoners in state prisons reported using marijuana at the time of their crime, up from 15 percent in 1997; 63 percent said they were regular users at the time of the crime, up from 58 percent in 1997. All the surveys, whether in the United States or Australia, and whether of new arrestees or long-term prisoners, showed much higher rates of marijuana use than of other drugs.
Taken together, this work implies that marijuana use may be as large or larger a risk for violence as drinking. At the least, it suggests the laboratory research on marijuana and aggression is flawed. Those studies often exclude people who have a history of psychotic disorders. Researchers cannot ethically expose them to cannabis given the known link between the drug and psychosis. And older studies were conducted with marijuana much lower in THC than the high-potency strains smoked today.
As a result, the laboratory work misses the most devastating kind of violence that marijuana provokes.
Alcohol has predictable effects. It disinhibits drinkers and makes them more aggressive, whatever their baseline. Marijuana doesn’t cause aggression in everyone. Many users relax. But some become paranoid, and some of those have full-blown psychotic episodes.
Marijuana causes paranoia and psychosis.
That fact is now beyond dispute. Even scientists who aren’t sure if marijuana can cause permanent psychosis agree that it can cause temporary paranoia and psychotic episodes. The risk is so obvious that marijuana dispensaries advertise certain strains as less likely to cause paranoia.
Paranoia and psychosis cause violence.
Overwhelming evidence links psychotic disorders and violence, especially murder. Studies have confirmed the connection, across cultures, nations, races, and eras.
The definitive analysis was published in PLOS Medicine in 2009. Led by Seena Fazel, a psychiatrist and epidemiologist at Oxford University, researchers examined twenty earlier studies on people with schizophrenia and other forms of psychosis. They found that people with psychosis were 5 times as likely to commit violent crimes as those without it. They were 19.5 times as likely to commit murder. (Amazingly, a 2010 paper in the American Journal of Psychiatry on the deaths of young children in Taiwan found almost exactly the same risk ratio. Kids who had a parent with schizophrenia were 19.4 times as likely to die by homicide as those who did not.)
Full-blown schizophrenia is relatively rare. But because people with schizophrenia are so likely to kill, they make up an appreciable fraction of murderers—5 to 10 percent in most studies.
A 2007 British government study of all 5,884 people convicted of murder in England and Wales in the previous decade found that 348 of the perpetrators, or 6 percent, had a schizophrenia diagnosis. The rates of convictions of people with schizophrenia increased faster than the overall homicide rate over the period.
A broader analysis in 2009 of eighteen international studies found that people with schizophrenia were responsible for 6.5 percent of homicides—about 1 in every 15. Some more recent studies find even higher rates. A 2011 paper by Australian researchers examined 435 homicide cases and found that 9 percent of the killers had a schizophrenia diagnosis.
When researchers include forms of psychosis other than schizophrenia, they find an even stronger link. Studies by the Justice Department and other researchers show that 15 percent to 20 percent of American prisoners have diagnosable psychotic disorders. (The United States has about 2 million people in prison, so those estimates suggest that up to 400,000 American prisoners have psychosis. The number has soared in the last fifty years, as state mental hospitals have closed. Researchers and treatment advocates agree that many mentally ill people who would once have been housed in hospitals are now in prison.)
• • •
Marijuana causes psychosis.
Psychosis causes violence.
The obvious implication is that marijuana causes violence.
Yet the marijuana-psychosis-violence connection is even stronger than the A-causes-B, B-causes-C, therefore A-causes-C explanation suggests.
Here’s why.
The studies that demonstrate that psychosis causes violence also show that most of that violence occurs when people with psychosis are using drugs.
In other words, when a patient with schizophrenia stays on antipsychotic medicines and away from recreational drugs, he is only moderately more violent than a healthy person.
But many people with schizophrenia do not stay on their antipsychotics for long. And people with psychosis use and abuse drugs far more than the general population—and when they do, they become far, far more likely to commit violent crimes than healthy people are.
Fazel’s meta-analysis in 2009 that found people with psychosis had a fivefold increased risk of violence showed that the risk was tenfold when those people were also substance abusers. (It was about double for those who had psychosis but didn’t abuse drugs.)
Advocates for the mentally ill put great emphasis on the fact that people with psychosis aren’t overly violent if they don’t use drugs. But they rarely acknowledge the flip side of the issue, that people with psychosis are frequently drug abusers—and that as a result their overall risk for violence is very high.
“You don’t just have an increased risk of one thing—these things occur in clusters,” Fazel told me over a coffee in London’s hip Borough Market; he’d come in from Oxford for a conference, and I was on my way to the Institute of Psychiatry and Robin Murray. “You have a set of genes that cluster around the difficult things.” The difficult things. Like a tendency to murder. The understatement seemed very British.
And the drug that mentally ill people use the most is marijuana. Unfortunately, its tendency to cause paranoia and psychosis makes it a terrible choice.
A 2018 paper in the journal African Health Sciences examining 520 psychotic patients admitted to a South African psychiatric hospital in 2012 and 2013 found that 49 percent had engaged in violent behavior during their psychotic episodes. Of those, most had used cannabis, sometimes alongside methamphetamine.
The 2007 British government survey of homicides found that almost 60 percent of the 348 offenders with schizophrenia were using drugs when they killed. Cannabis accounted for most cases, with cocaine and amphetamine almost all the rest—not surprising, because stimulants can also cause psychosis. The number of murderers who were both psychotic and drug abusing increased far faster than the overall homicide rate over the study period.
It is worth remembering that—in part, thanks to Robin Murray and the Institute of Psychiatry—cannabis use in Britain peaked between 2000 and 2005. Since then, cannabis use has fallen, and the number of homicides committed by people with schizophrenia has also trended lower, according to a more recent British government report.
Few studies have directly examined the interplay between marijuana, mental illness, and violence in depth. Until last June, the most striking came from the Dunedin research. Written by Louise Arseneault and published in 2000 in the Archives of General Psychiatry, it showed that people with marijuana dependence had a fourfold risk for violence, even after adjusting for other variables. (People with alcohol dependence had twice the risk, so the Dunedin study is another that demonstrates marijuana is more correlated with violence than alcohol is.) Patients with schizophrenia or schizophrenia spectrum disorders had 2.5 times the risk of the average person.
But the most stunning figure covered people who had both a cannabis disorder and a schizophrenia spectrum diagnosis. Those people were 18 times as likely to commit violence as the average person. Even that figure may understate the connection between marijuana, mental illness, and murder. Because the Dunedin study was so small, it used broad measures for violence. But Fazel’s 2009 paper showed that the more severe the crime, the higher the risk that a psychotic person would commit it.
In other words, in people predisposed to delusions, marijuana seems to function as a supercharger for sudden, extreme violence. Their underlying fears make them prone to lash out uncontrollably if cannabis provokes their paranoia. In a study published in the Medical Journal of Australia in 2006, of 88 murderers with psychosis in Australia, 50 reported delusional beliefs that the victim was threatening them or needed to be killed to save other people; 52 of the killers reported being dependent on marijuana.
In 2013, researchers in Italy examined violence in 1,582 psychiatric patients in Southern Italy and found another strong link. In their paper, which was published in Rivista di psichiatra—Journal of Psychiatry—they reported drug users were almost 9 times as likely to become violent as patients who didn’t use. Eighty percent of drug users “exhibited violent behavior,” and cannabis was the most frequent drug of abuse.
Even after adjusting for age, gender, and type of psychiatric illness, using marijuana caused a roughly four-fold increase in the risk of violence leading to injuries, the researchers reported. “Cannabis use/abuse is associated with violent behavior inflicted on the self and others, and constitutes a specific risk factor.” The researchers also noted the unpredictability of violence associated with cannabis. “Violent behavior was correlated only to cannabis use/abuse and had a tendency to recur, being ‘immediate’ and therefore difficult to predict.”
Then, last June, Swiss psychiatrists examined violent behavior among 240 young psychotic patients in their clinics.
Over a three-year period, 62 of the 240 became violent, which researchers defined as “an assault causing any degree of injury, any use of a weapon or any sexual assault.” Three patients attempted or committed murder.
In other words, these patients had a 1-in-4 chance of becoming violent over a three-year period—even though they were in treatment, and even though Switzerland has low crime rates overall.
The researchers then looked at the risks for violence. In a 2017 paper in the journal Early Intervention in Psychiatry, they examined various potential causes, including substance abuse—though they didn’t specifically focus on cannabis. Patients who lacked insight into their psychosis were more violent, they found. But that finding wasn’t a huge surprise, and the differences were relatively small.
Then the researchers reanalyzed the data, this time comparing the 82 patients who were dependent on cannabis with the 158 who were not. Their new paper appeared online in Frontiers in Psychiatry on June 14, 2018. It found an extraordinary association between cannabis and violence.
Psychotic patients with a cannabis use disorder had a nearly 50 percent chance of committing violence during the three years of the study. Those who weren’t using had only a 15 percent chance. When researchers adjusted for other variables, such as alcohol use or adherence to treatment, the gap increased. The cannabis-dependent patients were four times as likely to be violent. No other factor was nearly as important. Alcohol use, which was common among the patients, made no difference.
I emailed Valerie Moulin, the psychiatrist who led the study, to ask why she had decided to check specifically for the effects of marijuana. “At first I wasn’t particularly looking for this link between cannabis and violence,” she wrote back. “I was more interested in poly-consumption (alcohol and cannabis, in particular), because we thought it was what was relevant.”
But her experiences with patients—along with discussions with other psychiatrists and the handful of recent papers showing the cannabis-violence risk—led her to look again. “We were surprised by this link,” she wrote. “Now we need to look at it to try to minimize violence.”
Moulin encouraged other researchers looking at violence and psychosis to split out cannabis rather than lumping it with other illicit drugs. “It was a mistake to mix everything up, and it overshadowed the understanding of the effects of cannabis.”
But much more work remains to be done, Moulin wrote. She will continue her research to see if she can tease out why cannabis-using patients are so prone to violence—and whether the risk increases over time.
Moulin added that she believes marijuana’s tendency to cause violence probably occurs not only in patients with preexisting psychosis but in otherwise healthy people. “The trend is towards a major effect of cannabis use,” she wrote.
Harry Anslinger might have been a racist jerk, but eighty-five years ago, he was right about marijuana.