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On December 19, 2014, in Cairns Australia Raina Thaiday stabbed eight children to death. She was related to all the children. “Seven of the kids were hers. The eighth was her niece.” On April 6, 2017, a Justice of the Supreme Court of Queensland heard testimony from Thaiday’s psychiatrists and later determined she couldn’t control her actions and had broken from reality when she killed her children. “In medical terms, she suffered from psychosis and the devastating mental illness schizophrenia, which can cause hallucinations, delusions, and paranoia.” Justice Dalton ruled that marijuana had caused her mental illness.

“Thaiday gave a history of the use of cannabis since she was in grade 9,” Dalton wrote. “All the psychiatrists thought that it is likely that it is this long-term use of cannabis that caused the mental illness schizophrenia to emerge.” With those words, Dalton made one of the first judicial findings anywhere linking marijuana, schizophrenia, and violence—a connection that cannabis advocates are desperate to hide.

The above anecdotal story was described in Tell Your Children: The Truth About Marijuana, Mental Illness, and Violence, by Alex Berenson. He noted the association of cannabis and violence dated back to around 100 AD. A Chinese pharmacopeia called the Pen-ts’ao Ching, warned how cannabis “stimulate(d) uncontrollable violence and criminal inclinations.” More recently, in 2017 the National Academy of Medicine issued a research report titled, “The Health Effects of Cannabis and Cannabinoids.” The report concluded there was substantial evidence of an association between cannabis use and the development of schizophrenia or other psychoses. And the highest risk was among the most frequent users.

The association between cannabis use and the development of a psychotic disorder is supported by data synthesized in several good-quality systematic reviews. The magnitude of this association is moderate to large and appears to be dose-dependent, and it may be moderated by genetic factors. Factors contributing to the strength of the evidence derived from the cited systematic reviews include large sample sizes, the relative homogeneity of the findings, the presence of relationships between the dose/exposure and the risk, the studies having been controlled for confounders, and the systematic reviews having assessed for publication bias. The primary literature reviewed by the committee confirms the conclusions of the systematic reviews, including the association between cannabis use and psychotic outcome and the dose-dependency of the effects, further bolstering the overall strength of evidence for our conclusions.

Cascading social changes, with state-after-state legalizing medical and then recreational marijuana has broadened its use beyond the subculture of jazz musicians who smoked it in the 1920s and 1930s. Berensen noted the number of emergency room visitors diagnosed primarily with psychosis and secondarily with marijuana-related problems tripled between 2006 and 2014 from 30,000 to 90,000. “By 2014, 11 percent of Americans who showed up in emergency rooms with a psychotic disorder also had a secondary diagnosis of marijuana misuse.” Studies from Denmark and Finland have shown recent increases in schizophrenia diagnoses coming after rising cannabis use. The authors of the Denmark study said: “The increase in cannabis-induced psychosis follows both the increase in the level of THC in cannabis, and the increase in cannabis use.”

Marijuana is an intoxicant that can disinhibit users, too. And though it sends many people into a relaxed haze, it also frequently causes paranoia and psychosis. Sometimes those are short-term episodes in healthy people. Sometimes they are months-long spirals in people with schizophrenia or bipolar disorder.

There is also a correlation between marijuana and violence, despite the denial of marijuana proselytizers. All four states that legalized marijuana in 2014 and 2015—Alaska, Colorado, Oregon and Washington—have seen sharp increases in murders and aggravated assaults. The four states combined have an increase in murders of 25% and an increase in assaults of 35% between 2013 and 2017. That compares nationally to an increase in murders of 20% and in aggravated assaults of 10%. There is only an association here; it is correlative, not causative. “Knowing exactly how many of these crimes are related to marijuana is impossible without researching each of them in detail, but police reports and arrest warrants show a clear connection in many cases.”

There is also a genre of scholarly studies linking marijuana and psychosis. Jim van Os, a Dutch psychiatrist and epidemiologist was the lead author of a 2002 study, “Cannabis use and psychosis: A longitudinal population-based study,” that found a history of cannabis use increased the risk of a follow-up psychosis outcome for subjects with no history of psychosis. “A baseline lifetime history of cannabis use was a stronger predictor of psychosis outcome than was use over the follow-up period and use of other drugs.” There was an additive interaction between cannabis use and the risk of psychosis. “The difference in risk of psychosis at follow-up between those who did and did not use cannabis was much stronger for those with an established vulnerability as baseline than for those without one.” Berensen said Os’s survey of Dutch adults indicated adult users of marijuana were far more likely to develop psychosis than nonusers.

Benjamin Murrie and others did a study of the proportion of people with substance-induced psychoses who transition to schizophrenia. They found that the rate of transition to schizophrenia was higher after cannabis-induced psychosis (34%) than other substance-induced psychoses, including amphetamines (22%) and hallucinogens (24%). Their study found that substance-induced psychoses (especially cannabis-, hallucinogen- and amphetamine-induced psychoses) are associated with a significant risk of a later diagnosis of schizophrenia. And that risk is “only slightly less than that observed for some other brief psychotic episodes.”

Commenting on the study for Psychiatric Times, Brian Miller said the study’s findings have important implications for mental health services. Substance-induced psychoses are common reasons for individuals to seek help. Additionally, they are associated with a substantial risk of transition to schizophrenia. He said a key implication of the findings is that treating cannabis-, amphetamine-, and hallucinogen-induced psychoses should be considered in the same framework of intervention as other brief psychotic disorders, and may help decrease the rates of transition to schizophrenia.

Sir Robin MacGregor Murray and Marta Di Forti, leading experts on cannabis and psychosis, were part of a team of researchers in a study of first-episode psychosis attributable to the use of high potency cannabis, or “skunk.” Compared to those who never used cannabis, individuals who used skunk were nearly twice as likely to be diagnosed with a psychotic disorder if they used less than once per week, almost three times as likely if they used skunk weekends, and more than five times as likely if they were daily users. In a Reuters article on the study, Murray said: “Fifteen years ago, nobody thought that cannabis increased the risk of psychosis… But now the evidence is pretty clear.”

The results of our study support our previous conclusions from analysis of part of the sample; use of high-potency cannabis (skunk) confers an increased risk of psychosis compared with traditional low-potency cannabis (hash). Additionally, because of the increased sample size in the present study, we were able to combine information on frequency of use and type of cannabis used into a single measure. This combined measure suggested that the strongest predictor of case-control status (ie, predictor of whether a random individual would be case or control) was daily-skunk use.

Even the founder of NORML, Keith Stroup, knew that when marijuana was potent enough to get him high, it could make him paranoid. In his 1981 book, High in America, Patrick Anderson related how onetime Stroup “rocked so obsessively in a rocking chair that the chair broke.” Then he became convinced someone was about to murder his new daughter and raced home to save her. Another time after smoking pot, he became convinced that a friend who had come for dinner was going to kill them.

Case reports connecting marijuana and psychosis began appearing in the 1970s, as cannabis use surged. There was “Psychotic symptoms due to cannabis abuse,” in the National Library of Medicine, 1970; “Forty-six cases of psychosis in cannabis abusers,” in International Journal of the Addictions, 1972; “Psychotic reactions following cannabis use in East Indians,” in Archives of General Psychiatry, 1974. “Cannabis-associated psychosis with hypomanic features” appeared in Lancet in 1982. But these were only case reports.

Sven Andréasson was able to demonstrate a statistical link between marijuana and schizophrenia for the first time in, “Cannabis and schizophrenia: A longitudinal study of Swedish conscripts,” Lancet, 1987. Andréasson did a follow up study to examine if people who developed schizophrenia after smoking marijuana were different than schizophrenic patients who had never smoked, “Schizophrenia in users and nonusers of cannabis,” Acta Psychiatrica Scandinavica, 1989. Berenson reported Andréasson found that marijuana smokers tended to be relatively high-functioning before their illness, while nonsmokers had a more classic history representative of schizophrenia—they were troubled from a much younger age.

Based on his data and later findings, Andréasson says he believes that cannabis is responsible for between 10 percent and 15 percent of schizophrenia cases. Few people develop schizophrenia solely because of smoking, he thinks. But many who would not have become sick do so because marijuana pushes their vulnerable brains over the edge.

Then there was a longitudinal study of individuals from Dunedin, New Zealand: “Cannabis use in adolescence and risk for adult psychosis,” in the British Medical Journal, 2002. The authors found cannabis use in adolescence increased the likelihood of experiencing symptoms of schizophrenia in adulthood. Individuals who used cannabis by the age of 15 were 4 times as likely to have a diagnosis of schizophreniform (a psychotic illness with schizophrenia-like symptoms lasting less 6 months). Their findings agreed with the Andréasson study, and added three new pieces of evidence.

Firstly, cannabis use is associated with an increased risk of experiencing schizophrenia symptoms, even after psychotic symptoms preceding the onset of cannabis use are controlled for, indicating that cannabis use is not secondary to a pre-existing psychosis. Secondly, early cannabis use (by age 15) confers greater risk for schizophrenia outcomes than later cannabis use (by age 18). The youngest cannabis users may be most at risk because their cannabis use becomes longstanding. Thirdly, risk was specific to cannabis use, as opposed to use of other drugs, and early cannabis use did not predict later depression.

Along with the van Os study, described above, the Andréasson study and the Dunedin study provide significant and powerful evidence linking cannabis and mental illnesses, such as schizophrenia and depression. They all reached the same conclusion, came at the question of marijuana and psychosis from three different directions and were from three different countries. An editorial in the BMJ, “Cannabis and mental health,” discussing these three studies said:

Although the number of studies is small, these findings strengthen the argument that use of cannabis increases the risk of schizophrenia and depression, and they provide little support for the belief that the association between marijuana use and mental health problems is largely due to self-medication.

The connection between marijuana and madness harkens back to an education-exploitation film called Reefer Madness or Tell Your Children. Reefer Madness is a melodramatic, 1936 film that attempted to show the dangers of marijuana use. It became a satirical favorite among advocates for marijuana reform in the 1970s when it was used in an attempt to raise support for the California Marijuana Initiative in 1972. Keith Stroup found a copy in the Library of Congress archives and bought a print for $297. Robert Shaye of New Line Cinema saw the film, realized it was in the public domain, and began distributing the film nationally.

In the film, Ralph, who is a dealer, becomes paranoid from his use of marijuana (and he also accidentally killed a teenaged girl). The Boss eventually sends someone to kill Ralph, who struggles with the hitman and beats the assassin to death with a stick. Ralph is then arrested and sent to an asylum for the criminally insane, “for the rest of his natural life.” The connection between marijuana and madness seems to becoming less campy satire and more reality. For more on the film, see “Remembering Reefer Madness.”