Cannabis Inquiry

With the upcoming Referendum on legalisation of cannabis in 2020, we ask that the government first investigate the possible link between cannabis and violence.

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Over the past couple of decades, studies around the globe have found that THC – the active compound in cannabis – is strongly linked to psychosis, schizophrenia, and violence. A certain percentage of people who use marijuana can become psychotic and violent. Researchers have studied alcohol and violence for generations, proving that alcohol is a risk factor for domestic abuse, assault and even murder. Far less work has been done on cannabis.

It raises an important question – is our relatively higher use of cannabis compared to other countries related to our horrific record when it comes to child abuse and family violence?

In 2018, researchers at Ohio and Tennessee Universities found that marijuana use was positively and significantly associated with psychological, physical, and sexual intimate partner violence, after controlling for alcohol use and problems, antisocial personality symptoms, and relationship satisfaction. The researchers say that treatment of men arrested for domestic violence should consider reducing their marijuana use.

Research published in 2016 in the journal Psychological Medicine concluded that continued use of cannabis causes violent behaviour as a direct result of changes in brain function that are caused by smoking weed over many years. The results showed that continued cannabis use is associated with 7-fold greater odds for subsequent commission of violent crimes.

A University of Florida study published in The Journal of Interpersonal Violence in 2011 found that frequent marijuana users in adolescence are twice as likely to engage in domestic violence as young adults.  The same study showed this group were more than twice as likely to become a victim of domestic violence. The researchers said “These findings have implications for intimate partner violence prevention efforts, as marijuana use should be considered as a target of early intimate partner violence intervention and treatment programming.”

A 2007 paper in the Medical Journal of Australia looked at 88 defendants who had committed homicide during psychotic episodes. It found that most of the killers believed they were in danger from the victim, and almost two-thirds reported misusing cannabis – more than alcohol and amphetamines combined.

A 2002 study in BMJ (formerly the British Medical Journal) found that people who used cannabis by age 15 were four times as likely to develop schizophrenia or a related syndrome as those who’d never used. Even when the researchers excluded children who had shown signs of psychosis by age 11, they found that the adolescent users had a threefold higher risk of demonstrating symptoms of schizophrenia later on. The study was based on the Dunedin multidisciplinary health and development study.

The United Nations Office on Drugs and Crime (UNODC) summed up the issue in their 2012 report, saying that THC content and the potency of cannabis have been increasing over the past 30 years, and that this can increase psychotic symptoms in regular users.

Some evidence is already appearing in New Zealand. Last year (2018), a man who stalked several women during a 24-hour drug-induced psychosis has left one of his victims with ”a lasting fear”. He lost his job after failing a drug test and then embarked on a four-day cannabis binge. The judge said that resulted in a psychosis. In 2017, a man repaid a family who had taken him in by stabbing the mother, a babysitter and their pet dog in a drug-induced rage. The judge said that his consumption of cannabis, which may have been laced unknowingly with methamphetamine, had caused him to attack.

Earlier this month in California, Bryn Spejcher, an employed, well-educated 28-year-old with no criminal record or history of mental illness appeared in court accused of stabbing her boyfriend to death — after smoking pot for the first time. The coroner testified that the victim had been stabbed 108 times, from his head to his knees, cutting his trachea, jugular vein and carotid artery and perforating his heart twice. A forensic scientist from the crime lab confirmed that no drug other than THC was present in Bryn’s blood and no drug other than THC was found in the bong.

Paranoia and psychosis can make some people dangerous, so a rising use of a drug that causes both would be expected to increase violent crime, rather than reduce it as drug advocates might claim.

Of the four US states that legalised marijuana in 2014 and 2015 – Oregon, Washington, Alaska, and Colorado – there was a combined 35% increase in murders in those states from 2013 to 2017, compared with a 20% rise nationally. Washington became the first U.S. jurisdiction to legalise recreational marijuana in 2014. Between 2013 and 2017, the state’s aggravated-assault rate rose 17%, which was nearly twice the increase seen nationwide, and the murder rate rose 44%, which was more than twice the increase nationwide.

Here in New Zealand, we know from a number of governmental reports (UNICEF reports in 2003 and 2007, a CYF report in 2006, and a Children’s Commissioner report in 2009) that one of the factors most commonly associated with the maltreatment of children is drug abuse.

In March, Texas released its report on child abuse deaths, finding half the 172 child abuse deaths in 2017 coupled with substance abuse.  Marijuana was the most-used substance connected to child abuse and neglect deaths, followed by alcohol, cocaine and methamphetamine. In one terrible case last year, Cynthia Randolph left her 1-year old and 2-year-old in the car while she smoked pot. Both children died.

In 2017, Arizona also published a report showing that marijuana was the substance most often linked to child abuse deaths in 2016.


just-published study in The Lancet concluded that “people who smoked marijuana on a daily basis were three times more likely to be diagnosed with psychosis compared with people who never used the drug. For those who used high-potency marijuana daily, the risk jumped to nearly five times.”

But it is the most recent study that we should sit up and take notice of. The paper “Cannabis use and violence in patients with severe mental illnesses: A meta-analytical investigation” published last month is the most comprehensive survey yet on the issue. Findings showed a moderate cannabis-violence association in severe mental illness. What’s also striking is how recent most of the papers examined are – 10 of the 12 papers are in the last decade, and 7 of the 12 since 2016.

Just this month, more than 40 clinicians, researchers, scientists, and other public health professionals from Massachusetts, including many from Harvard Medical School, released a Statement of Concern, highlighting negative effects of THC, including “Increased risk of serious mental health problems including acute psychosis (e.g., hallucinations, delusions), paranoia, schizophrenia, depression, anxiety, and suicide, with growing scientific evidence that daily use of high THC products bring greater risk”. They highlight 2018 research from the Copenhagen University Hospital which found that “41% of those who experience cannabis-induced psychosis later convert to schizophrenia.”

Last month in the liberal state of Vermont, the Department of Mental Health has warned legislators about the mental health implications of a commercial market, stating, “Perhaps the strongest evidence for severe mental health problems related to cannabis use is related to psychosis were multiple studies have linked regular cannabis use to an estimated doubling of the risk of a psychotic illness as well a more refractory course among people with existing psychotic illness. Violent behavior as a result of cannabis induced paranoia and other psychotic symptoms is also an increasing concern.”

 In Maryland, neuroscientist and author of “The Impact of Marijuana on Mental Health in: Contemporary Health Issues on Marijuana” Christine Miller warned legislators last month, “The causal link between marijuana use and the development of psychosis is quite simply the most well-replicated, high-impact finding in schizophrenia research today. Given current use rates and the strong potency of the drug available, it stands to be responsible for a larger proportion of schizophrenia cases than any other established factor. Who may be at risk cannot be reliably predicted.” 


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