Marijuana is known to produce different results in different people. It has calming effects on some and causes paranoia in others. A new study links heavy marijuana use and schizophrenia in men. Read on to find out what the research reveals.

About the Study That Links Heavy Marijuana Use and Schizophrenia

Marijuana has long been looked at for its psychoactive effects. French psychiatrist Jacques-Joseph Moreau published a book called Hashish and Mental Illness back in 1845. In it, he described his experiences with cannabis, which he previously referred to as a “marvelous substance” bringing “indescribable delights”. But he also detailed how it can bring on feelings of psychosis when consumed in large quantities.

Now, almost 180 years later, we are discovering that the author was on to something. A Danish study published in the journal Psychological Medicine provides evidence of a link between heavy marijuana use and schizophrenia, particularly among young males.

The study is likely to be the largest of its kind. It focused on the relationship between cannabis and psychosis. It looked at health histories recorded between 1972 and 2021 examining the records of 6.9 million people. It found that up to 30% of schizophrenia diagnoses, about 30,000 in total, could have been avoided if men 21 to 30 years old, had not developed a cannabis use disorder.

Research in broader age ranges between 16 and 49 show that schizophrenia could be avoided in 15% of men and 4% of women if heavy marijuana use had not been a factor.

The study does not offer absolute proof of a link between heavy marijuana use and schizophrenia. Randomized controlled trials would be needed to establish evidence. But it is supported by a direct relationship between an increase in schizophrenia diagnoses and a rise in THC use and potency.

“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 Hjorthoj, 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.

More Details About the Study Linking Heavy Marijuana Use and Schizophrenia

Gender and age were considered in the study outcomes.

“We found that the proportion of cases of schizophrenia that were attributed to cannabis use disorder, and those that might have been prevented, was much higher in males than females, and, in particular, younger males in whom the brain is still maturing. And we saw that this increase was taking place over time, completely in parallel with the increasing potency of cannabis,” said Hjorthoj.

The size of the study makes its outcome something to consider.

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“This is the first time we’ve seen a large-scale study across an entire population that addresses the relationship of cannabis and schizophrenia across different age and sex groups,” says Wilson M. Compton, deputy director of the National Institute of Drug Abuse (NIDA) which collaborated with the Mental Health Services in the Capital Region of Denmark on the study design.

The NIDA proposed that age and gender be analyzed after discovering earlier work by the hospital that explored the heavy marijuana use and schizophrenia link.

What the Heavy Marijuana Use and Schizophrenia Study Implies

Compton points out that the study raises concerns as to whether young men are more at risk of developing psychosis than females and how cannabis plays a role in mental health effects.

It may also provide guidance in prevention and treatment.

“People are their own agents. They can decide for themselves. But they should, if they do use cannabis, decide based on proper data and not from a story that cannabis is completely harmless and maybe even something everybody should use, which I think is the way public discourse is moving,”Hjorthoj points out.

Other Studies and Opinions on the Heavy Marijuana Use and Schizophrenia Link

Other studies have looked at the heavy marijuana use and schizophrenia link suggesting an alternate explanation. Carl Hart of Columbia University and Charles Ksir of the University of Washington have conducted similar research and argue that heavy cannabis use is a typical behavioral issue in people vulnerable to schizophrenia- along with drinking, smoking, and other types of drug use.

Marijuana buds with marijuana joints and Cannabis oil

“Future research studies that ‘put on blinders’ 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,” Hart and Ksir wrote in their study.

David Nutt, a professor of neuropsychopharmacology at Imperial College London and founder of the nonprofit Drug Science which provides information about drugs “free from political or commercial influence” calls the study “Intriguing” but notes that “It also raises many more questions”.

He wonders whether some of the study participants may have been misdiagnosed with schizophrenia which could have been cannabis-induced psychosis. He also entertained the possibility that the lower cases of schizophrenia among women may suggest that cannabis protects against mental illness.

He also pointed out that unconsidered factors such as how frequently the participants used marijuana and the potency of the cannabis they used may have swayed the results.

Robin Murray, a professor of psychiatric research at the Institute of Psychiatry King’s College London and co-editor-in-chief of Psychological Medicine notes that previous studies that examine the link between heavy marijuana use and schizophrenia have been criticized in the past.

But he points out that this study considered gender and age making the connection more causal.

“It is currently impossible to prove 100 percent definite causal link between any environmental factor and schizophrenia because we do not have an animal model of schizophrenia,” Murray notes.

“Epidemiology showed a relationship between tobacco smoking and cancer. The proof came from showing that painting tobacco tar on the skin of mice produced tumors. Unfortunately, we don’t know what we would have to induce in a rodent to prove that cannabis could cause schizophrenia,” he went on to say.