A huge Danish study shows that up to 30 percent of psychosis diagnoses in young men could have been prevented if these individuals hadn’t used marijuana heavily
The French psychiatrist Jacques-Joseph Moreau published a book called Hashish and Mental Illness in 1845, the same year that Scientific American brought out its first issue. In it, he explored his own experiences with the drug cannabis at the Paris-based Club des Hachichins—some of which took place alongside the likes of Victor Hugo, Honoré de Balzac and Charles Baudelaire. Two years earlier Moreau described the “undescribable delights” of a “marvelous substance.” But in his 1845 book, he noted in great clinical detail the similarities to psychosis when ingesting high doses.
Nearly two centuries later the possible link between cannabis use and schizophrenia continues to be intensively researched, often provoking heated debate in its wake. A study published on May 4 in the journal Psychological Medicine provides new evidence that problematic cannabis use may lead to schizophrenia, particularly for young men who are heavy users.
The research, likely the largest epidemiological investigation conducted to date that directly focused on the cannabis-psychosis question, delved into Danish health histories from 1972 to 2021. The study examined health records of 6.9 million people and found that up to 30 percent of schizophrenia diagnoses—about 3,000 in total—could have been prevented if men from 21 to 30 years old had not developed cannabis use disorder. The comparable prevention percentages for the broader age range of 16 to 49 were 15 percent for men and 4 percent for women.
The Danish epidemiology study does not offer hard-and-fast proof of the cannabis-schizophrenia connection, which could be accomplished only through randomized controlled trials. But this link is supported by the fact that marijuana use and potency have risen markedly—from 13 percent THC content in Denmark in 2006 to 30 percent in 2016—alongside a rising rate in schizophrenia diagnoses. “While this isn’t proving causality, it’s showing that the numbers behave exactly the way they should, under the assumption of causality, says Carsten Hjorthøj, the study’s lead author and an associate professor at the Mental Health Services in the Capital Region of Denmark and the University of Copenhagen.
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