French psychiatrist Jacques-Joseph Moreau published a book titled Hashish and mental illness in 1845, the same year as American Scientist released its first issue. In it, he explored his own experiences with the drug of cannabis at the Club des Hachichins in Paris, some of which took place alongside Victor Hugo, Honoré de Balzac and Charles Baudelaire. Two years earlier, Moreau described the “indescribable delights” of a “wonderful substance”. But in his 1845 book he noted in great clinical detail the similarities to psychosis when ingesting large doses.
Nearly two centuries later, the possible link between cannabis use and schizophrenia continues to be the subject of intensive research, often sparking heated debates in its wake. A study published on May 4 in the journal Psychological medicine provides new evidence that problematic cannabis use can lead to schizophrenia, especially in young men who are heavy users.
The research, possibly the largest epidemiological investigation conducted to date that focused directly on the issue of cannabis-psychosis, delved into Danish medical histories from 1972 to 2021. The study examined the health records of 6,9 millions of people and revealed that up to 30% of schizophrenia diagnoses – about 3,000 in total – could have been prevented if men aged 21 to 30 had not developed drug use disorder. cannabis. Comparable prevention percentages for the broader age group 16-49 were 15% for men and 4% for women.
The Danish epidemiological study does not offer irrefutable proof of the cannabis-schizophrenia link, which could only be accomplished by randomized controlled trials. But this link is supported by the fact that the use and potency of marijuana has increased dramatically – from 13% THC content in Denmark in 2006 to 30% in 2016 – alongside an increase in the rate of schizophrenia diagnoses. “Although this does not prove causation, it does show that the numbers behave exactly as they should, under the causality hypothesis, says Carsten Hjorthøj, lead author of the study and associate professor at the Mental Health Services of the Denmark’s Capital Region and the University of Copenhagen.
The researchers’ dive into a country’s health statistics probed gender and age-related risks. “We found that the proportion of schizophrenia cases attributable to a cannabis use disorder, and those that could have been prevented, were much higher in men than in women and, in particular, in older men. young people in whom the brain is still maturing,” Hjorthøj says. “And we’ve seen that increase happening over time, completely parallel to the growing potency of cannabis.”
The size of the study could amplify its impact. “This is the first time we’ve seen a large-scale, whole-population study that addresses the relationship between cannabis and schizophrenia across different age and gender groups,” says Wilson M. Compton, deputy director of the National Institute on Drug Abuse. (NIDA), which collaborated with Mental Health Services in the Capital Region of Denmark to design the study. NIDA officials proposed the age and sex analysis after discovering previous work by the Danish hospital exploring the link between cannabis and schizophrenia.
The Danish-funded study, Compton says, raises a number of questions for future research as to whether adolescent male brains are at greater risk than female brains of developing marijuana-induced psychosis or whether levels of men’s exposure to cannabis may explain the difference.
The study also has implications for public prevention and treatment strategies. “People are their own agents,” says Hjorthøj. “They can decide for themselves. But they should, if they use cannabis, decide on the basis of proper data and not a story that cannabis is completely harmless and maybe even something everyone everyone should be using, which I think is how public discourse evolves.
This latest research is unlikely to alter the longstanding debate surrounding cannabis, schizophrenia, and statistical cause-and-effect relationships. Other researchers – Carl Hart of Columbia University and Charles Ksir of the University of Wyoming – have already suggested another explanation for the link. In a 2016 review article, they argued that heavy cannabis use is part of a set of problematic behaviors that manifest in some young people who may be vulnerable to schizophrenia. These young people may consume not only a lot of cannabis, but also cigarettes, alcohol and other drugs, while neglecting their schoolwork, all behaviors which, in sum, could contribute to a higher risk of diagnosis of psychosis or other mental disorder. “Future research studies that ‘put blinders on’ and focus exclusively on the cannabis-psychosis association will therefore not be of much value to us in our efforts to better understand psychosis and how and why it occurs” , wrote the authors.
David Nutt, professor of neuropsychopharmacology at Imperial College London and founder of the nonprofit Drug Science, which provides drug information “without political or commercial influence”, calls the Danish study “intriguing “. But, he adds, “it also raises many other questions.”
Specifically, Nutt asks if some of the cases in the study may have been misdiagnosed with schizophrenia — rather than an alternative diagnosis of another condition, cannabis-induced psychosis. He wonders if the lower reported risk figures for women suggest that cannabis might have a protective effect against schizophrenia. It also points to factors recognized by the researchers that could potentially skew the results, such as a lack of data on participants’ frequency of cannabis use or age at first use or amount of THC, the ingredient psychoactive of cannabis, in products. They have used.
The debate will no doubt continue. NIDA’s Compton suggests that prevention and education programs that warn of cannabis-related risk could be a way to test whether the alleged link between cannabis and schizophrenia has some merit. “Scientifically, if you can change cannabis use rates, it will test the theory that cannabis causes schizophrenia,” he says. “And so one would assume, if you reduce cannabis use, that the rates would go down.”
Robin Murray, Professor of Psychiatric Research at the Institute of Psychiatry at King’s College London and co-editor of psychological medicine, acknowledges that studies linking problematic cannabis use to schizophrenia have been criticized for their correlation. But the Danish study, he says, examines more closely than previous efforts the specific factors – gender and age – involved in the possible link between the drug and the disease. This adds to the growing body of research that has gradually ruled out alternatives to cannabis as a trigger for schizophrenia, making this link increasingly plausible. “So the causal effect is almost certain,” Murray says.
“It is currently impossible to prove a 100% definite causal link between an environmental factor and schizophrenia,” he adds, “because we don’t have an animal model of schizophrenia.”
“Epidemiology has shown a relationship between smoking and cancer. The evidence came from showing that painting tobacco tar on the skin of mice produced tumors,” says Murray. “Unfortunately, we don’t know what it would take to induce a rodent to prove that cannabis can cause schizophrenia.”