Is There a Link Between Marijuana Use and Psychosis?

Schizophrenia linked to marijuana use disorder is on the rise, study finds

Whiles browsing the internet, you are likely to see thousands of sites claiming that cannabis is harmless and even beneficial. 

However, a new study from Denmark indicates that the proportion of schizophrenia cases linked with problematic use of marijuana has increased over the past 25 years.

In 1995, 2% of schizophrenia diagnoses in the country were associated with cannabis use disorder. In 2000, it increased to around 4%. Since 2010, that figure rose to 8%, the study found.

“I think it is imperative to use such studies to highlight and emphasize that cannabis use is not harmless,” said Carsten Hjorthøj, an associate professor at the Copenhagen Research Center for Mental Health and an author of the study published in the medical journal JAMA Psychiatry.

“There is, unfortunately, evidence to suggest that cannabis is increasingly seen as a somewhat harmless substance. This is unfortunate since we see links with schizophrenia, poorer cognitive function, substance use disorders, etc.,” Hjorthøj wrote.

Cannabis use may be linked with suicidal thoughts

Previous research has suggested that schizophrenia is heightened for persons who use cannabis, and the association is mainly driven by heavy use of the drug. Many researchers hypothesize that cannabis use may be a “component cause,” which interacts with other risk factors to cause the psychiatric condition.

While the study was conducted in Denmark, Hjorthøj said, “I feel fairly confident that we will see similar patterns in places where problematic use of cannabis has increased, or where the potency of cannabis has increased.” We are aware that with the legalization of marijuana and better cultivation practices, the potency of cannabis available is now higher.

Around the world, tens of millions of people use cannabis. It’s legal for recreational use in 19 US states, Canada and several European countries. In these and some other places, it’s also approved to treat some medical conditions (though not as joints, as commonly believed!)

Single joint linked with temporary psychiatric symptoms

Cannabis use and cannabis use disorder have not only increased in Denmark; it’s a pattern seen globally. Recreational weed use is illegal in Denmark but is allowed for medicinal purposes.

Cannabis use disorder is usually defined as problematic use of the drug if the following aspects are observed: 

  • developing tolerance to weed
  • using cannabis in more significant amounts or over a more extended period than intended
  • being unable to reduce the use 
  • justifying the use
  • spending a lot of time procuring it, using or recovering from the effects of cannabis
  • giving up important activities and obligations in favour of cannabis 
  • and continued use of the drug despite negative consequences.

An increase in schizophrenia?

Schizophrenia is a chronic, severe and disabling mental disorder. Its symptoms may include delusions, thought disorders and hallucinations. Worldwide, schizophrenia affects over 20 million people. There is no ‘cure’, so psychiatrists try to manage the symptoms with medications and therapy.

While the study relates to Denmark, the picture is uncertain in other countries. In the US, the National Institute of Mental Health said it’s hard to obtain accurate estimates of the prevalence of schizophrenia because diagnosis is complex and overlaps with other disorders.

Many psychiatry textbooks state that the incidence of schizophrenia is constant over time and independent of geographical location.

Single joint linked with temporary psychiatric symptoms

The study included all people in Denmark born before December 31, 2000, who were 16 years or older from January 1, 1972, to December 31, 2016.

The findings could help explain the “general increase in the incidence of schizophrenia that has been observed in recent years” and provides some support that the “long-observed association between cannabis and schizophrenia is likely partially causal in nature,” the study said.

Legalization and regulation

The study assessed people who had a clinical diagnosis for cannabis treatment disorder, not general drug use, noted Terrie Moffitt, a professor and chair in Social Behaviour & Development of the Institute of Psychiatry, Psychology, and Neuroscience at King’s College London.

“This study of nationwide medical records adds important evidence that patients with diagnosed cannabis use disorder are more at risk for psychosis,” said Moffitt, who studies the effects of cannabis use on the mental health of the baby boomer generation.

However, Moffitt said that most cannabis users, even those dependent on it, never seek treatment and many people use it recreationally without developing problems.

“It is known that people who seek treatment tend to have multiple mental health problems, not solely cannabis problems,” Moffitt said. “And there are far more recreational cannabis users who manage cannabis well than cannabis-dependent users who cannot manage it.”

In an editorial that accompanied the study, Tyler J. VanderWeele, a professor in the Departments of Epidemiology and Biostatistics at the Harvard T. H. Chan School of Public Health, said that estimates are in the study are most likely to be conservative because of the underdiagnosis of cannabis use disorder.

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