The risk of developing schizophrenia is significant among individuals who have had drug-induced psychosis one or more times, a Norwegian study of Norwegian patients shows.

One in four people who have had cannabis-induced psychosis develop schizophrenia

People who have developed psychoses from getting high from cannabis have an increased risk of developing schizophrenia. The risk is much greater than if someone in your close family has had the diagnosis. Young men in particular are at risk.

Norwegian researchers have followed patients in Norway’s patient register for six years.

The goal was to see whether people who have had psychosis triggered by various drugs have an increased risk of developing a serious mental illness later on.

The results are striking.

People who have experienced a drug-induced psychosis have a significant risk of subsequently being diagnosed with schizophrenia, according to a recent Norwegian study.

Individuals who had a serious mental health diagnosis beforehand were excluded from the study, which was published in the American Journal of Psychiatry.

Found the same in Norway

Drug-induced psychosis is believed to be caused by a person’s drug use. It passes quickly as long as the patient does not take any more drugs.

But a significant proportion of these people later develop more long-term psychotic disorders, according to some international studies.

“We wanted to investigate whether we found the same relationship in the Norwegian data. And we did,” Eline Borger Rognli, a researcher at RusForsk at Oslo University Hospital, said to

She believes that these kinds of studies are important for understanding drug-induced psychoses.


  • Is not one specific disorder, but a sign or symptom combined with a state of confusion.
  • A person suffering from psychosis can hear voices that others do not hear or have sensory experiences that others do not experience. The feeling of being persecuted, having chaotic thoughts, persistent misunderstandings or inexplicable ideas are also common, such as believing that other people are spying on you or following you. Others may believe that their actions are influenced by signals from the TV or radio. Causes unrest and anxiety.
  • Symptoms can vary widely. Most people who receive treatment get better, and some recover completely.

(Source: Health Norway)

Cannabis psychosis and schizophrenia

The researchers checked whether drug-induced psychosis led to an increased incidence of either schizophrenia or bipolar disorder. Psychoses from various drugs such as cannabis, cocaine, amphetamines and alcohol were included in the study.

The researchers found the greatest risk of schizophrenia in people who developed cannabis-induced psychosis.

Among individuals who had had psychoses linked to drug use, 27 per cent were diagnosed with schizophrenia within six years.

The more frequently individuals had experienced drug-induced psychosis, the greater the risk was of being diagnosed with schizophrenia, regardless of gender.

Alcohol-induced psychosis presented the least risk.

Younger men most at risk

Among men, the risk was greatest among those who had psychosis triggered by cannabis or a number of different drugs. Younger men were most at risk.

“This agrees well with three previous studies which have also shown the clearest connection between cannabis-triggered psychosis and subsequent schizophrenia,” Jørgen Bramness, a specialist in psychiatry, said to

Bramness is a drug researcher at the Norwegian Institute of Public Health and one of the researchers behind the study.

Bramness is also a professor at UiT — the Arctic University of Norway in Tromsø and a researcher at the Norwegian National Advisory Unit on Concurrent Substance Abuse and Mental Illness (NKROP).

“The connection may be due to the use of drugs triggering a serious mental illness. But it can also point to the fact that people who develop psychosis as a result of drug use have a greater vulnerability to schizophrenia as well,” say researchers Eline Borger Rognli and Jørgen Bramness.

Some risk of bipolar disorder

The risk of being diagnosed with bipolar disorder after a drug-induced psychosis was much lower. Between four and five per cent of those who had a drug-induced psychosis later received this diagnosis.

Women had a higher risk than men of bipolar disorder after having a drug-induced psychosis.

The proportion who were later diagnosed with schizophrenia was six times higher than for manic attacks or bipolar disorder.


  • A collective term for a group of disorders with the same symptoms and disease course.
  • Symptoms can include delusions, hallucinations and dysfunction in everyday life (disorganization). Schizophrenics often have a flat affect, poor social functioning with low empathy. May also involve apathy, problems with memory and concentration.
  • The disorder is probably slightly more widespread among men than women. The majority become ill before the age of 30.
  • It is considered the most serious of the psychoses and is one of the most costly disorders in the Western world.
  • The reasons behind the disorder are complex, probably an interaction between genetics and environmental factors.
  • The risk of developing schizophrenia is around one per cent. Five hundred to 600 people are diagnosed annually in Norway. About three per cent of the population have the diagnosis at any given time.

(Source: Store norske leksikon)

Heredity as a risk factor ruled out

Researchers already know that schizophrenia involves a number of genes.

“The increased risk of getting schizophrenia after drug-related psychosis is even higher than what we find in first-degree relatives of those who have been given the diagnosis,” Bramness says.

First-degree relatives are parents and children.

The risk of being diagnosed with the malady for sons and daughters of a sick mother or father is "only" five to ten per cent.

In other words, there is no other risk factor for schizophrenia that is higher than related to drug-induced psychoses.

Trauma, infections and moving to another country are other known risk factors for schizophrenia, but these increase the risk to a much smaller degree.

Very low risk overall

Bramness emphasizes that although these factors increase the relative risk, the absolute risk of developing schizophrenia is very low.

Estimates of schizophrenia in the population vary. But it is estimated that approximately three per thousand of the population will develop schizophrenia.

“A doubled risk would mean that six per thousand of cannabis users would be predisposed, as an example,” Bramness said.

Very few develop drug-induced psychosis

Bramness is concerned that the study not be interpreted as painting a frightening picture of getting high on cannabis.

“The vast majority use cannabis in moderation and do not develop psychoses,” he said.

Very few people experience drug-induced psychosis, regardless of the drug.

“The risk of that is very small,” Bramness said.

But if you first develop psychosis after getting high, it is important to be aware of the risk and avoid drugs in the future.

“The most important thing this study says is that people who have had a psychotic episode are in the risk zone and should be followed up by the healthcare system,” he says.

Causal relationship is unclear

Even if there is a connection, it is complex.

“This may be because drug use triggers serious mental illness. But it also points to the fact that those who develop psychosis as a result of drug use are already highly vulnerable to schizophrenia,” Rognli said.

The researchers know that schizophrenia and other mental disorders often have a connection with more drug use. But they don't know if these people would develop psychoses anyway, even if they didn't get high, Bramness said.

In general, he thinks that calling a psychosis drug-induced is somewhat problematic.

“The risk these people have of getting schizophrenia probably already exists before they actually get high,” he explains.

Perhaps more likely to get high

Another factor that makes the picture somewhat unclear is that researchers do not know the amount of drugs the patients have consumed or how often they have used cannabis, for example.

“It’s conceivable that people who are vulnerable to schizophrenia tend to get high more often than the general population,” he said.

Bramness has conducted studies which show that people who are vulnerable to psychosis are also vulnerable to greater drug use.

“Maybe they use drugs to suppress symptoms as a kind of self-medication,” he said.

Agrees with previous research

Three previous studies have shown that the risk of developing schizophrenia was greatest after cannabis-induced psychosis, with between an 18 and 47 per cent increased risk.

The lowest risk was linked to alcohol-induced psychoses. There, the risk of a later schizophrenia diagnosis increased by five to 15 per cent.

A fourth study showed that cannabis, stimulant narcotics and opiates had a fairly similar increased risk of a subsequent diagnosis of schizophrenia if an individual had previously had a drug-induced psychosis.

This study also showed the lowest risk of serious mental illness in the case of alcohol-induced psychosis.

Here’s how the study was done

  • Researchers analysed admissions and diagnoses in the Norwegian Patient Register (NPR) between 2010 and 2015. All patients between the ages of 18 and 79 were included.
  • Patients who already had bipolar disorder or schizophrenia before experiencing a drug-induced psychosis were excluded.
  • The researchers then checked the proportion of patients who had had one or more psychoses triggered by drugs, who later received serious mental health diagnoses.
  • The study was carried out with researchers from the University of Tromsø, the University Hospital of Northern Norway, the Norwegian Institute of Public Health, the Innlandet Hospital and the Centre for Clinical Documentation.

Translated by: Nancy Bazilchuk

Read the Norwegian version of this article on


E. B. Rognli, I. H. Heiberg, B. K. Jacobsen, A. Høye and J. G. Bramness: Transition From Substance-Induced Psychosis to Schizophrenia Spectrum Disorder or Bipolar Disorder. American Journal of Psychiatry, 3 May 2023.


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