The link between cannabis and schizophrenia has always caused much ink to flow. From the 19the century, cannabis has been shown to trigger delusional episodes. The psychiatrist Jacques-Joseph Moreau (1804-1884) even studied the effects of insanity generated by the absorption of hashish.
A study published on May 4 in review Psychological Medicine confirms that excessive cannabis use increases the risk of schizophrenia, especially in young men. To do this, scientists from the Mental Health Services of Denmark and the American Institute for Addiction Research, the National Institute on Drug Abuseexamined the medical records of nearly 7 million Danes aged 16 to 49, from 1972 to 2021.
Very different from one person to another, schizophrenia, a complex pathology that results in a disturbed perception of reality, affects approximately 1% of the population. The symptoms are very variable – withdrawal, delusions, hallucinations, cognitive disorders…
The researchers confirm that the earlier and more frequent the consumption, the greater the risk. Admittedly, this work does not provide irrefutable proof of the link between cannabis and schizophrenia. But they observe that the consumption of cannabis has increased sharply in Denmark as well as its content in THC (delta-9-tetrahydrocannabinol), active ingredient, increased from 13% in 2006 to 30% in 2016, as did the number of diagnoses of schizophrenia in men.
They identified 45,327 cases of schizophrenia in total. By isolating cannabis users, it appears that 15% of cases of schizophrenia could have been prevented among men aged 16 to 49, and 4% among women aged 16 to 49, and up to 30% among men aged 21 to 30.
“Vulnerable period of 13 to 16 years”
“This Danish study is robust and carried out by a very strong team. It shows that young men seem the most vulnerable. It reinforces other work which has shown a very strong correlation between the use of cannabis and the presence of psychotic disorders.underlines the psychiatrist Jean-Michel Delile, president of Federation Addiction.
Thus, a Swedish study of 45,000 conscripts, published in The Lancet in 1987, showed that the more young people had used cannabis before joining the army, the greater the risk of developing a psychotic disorder. Likewise, a study conducted on a New Zealand cohort in 2002 showed that subjects who consumed cannabis in relatively large quantities and who, above all, had started before the age of 15, presented a markedly increased risk of psychotic disorder: multiplied by three for regular users and by ten for those who had used cannabis before the age of 15. Cannabis consumption precedes the onset of psychotic symptoms and not the other way round, recalls Jean-Michel Delile, in an article published in the journal Psychotropics in 2022.
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