Cannabis Not the Only Illicit Drug Linked to Schizophrenia

EDINBURGH, Scotland — Episodes of psychosis induced from several types of illicit substances are significantly linked to a later clinical diagnosis of schizophrenia, new research suggests.

The register-based study included more than 3000 inpatients from Scotland who experienced substance-induced psychoses. It showed that those who experienced cannabis- or polypharmacy-induced psychosis requiring hospitalization had the greatest increased risk of developing subsequent schizophrenia.

However, conversion rates to schizophrenia were also high for those who had had episodes of stimulant- and opioid-induced psychoses.

In addition, the investigators found that the majority of patients who converted to schizophrenia did so within 5 years of their substance-induced hospitalization.

"We did find that those who had cannabis-related psychosis or polypharmacy psychosis were more likely to develop schizophrenia than those who had other related psychosis," lead author David M. Semple, MBBS, MRCPsych, consultant psychiatrist from Hairmyres Hospital, East Kilbride, Glasgow, Scotland, told Medscape Medical News.

"This backs up a lot of previous work that showed that cannabis is an independent risk factor for schizophrenia. And it suggests that if you encounter someone with a cannabis-induced psychotic episode, the likelihood that they will develop schizophrenia is approaching 25%, or about 1 in 4," said Dr. Semple.

Nevertheless, he added that clinicians should follow patients "for quite some time" after a first episode of psychosis induced by any type of substance.

"There may be other genetic or predisposing factors, but if presented with this very severe episode that results in hospitalization, you'd be well advised to follow up for at least 5 years."

The preliminary results were presented here at the International Congress of the Royal College of Psychiatrists (RCPsych) 2013.

Significant Conversion Rates

"To date," said Dr. Semple, "few studies have examined the course of substance-induced psychosis." Also, when determining the likelihood of developing a chronic psychiatric disorder, most researchers have only used "broad definitions of schizophrenia spectrum disorders."
Dr. David Semple

For the current study, the investigators sought to assess the conversion rates from an episode of substance-induced psychosis to a clinical diagnosis of schizophrenia, as well as to evaluate time to conversion.

They examined data from the Scottish Morbidity Record on 3486 patients who were admitted to a psychiatric hospital between January 1997 and June 2012 for a first episode of substance-induced psychosis. All of these patients were then followed until July 2012 or until they experienced a first episode of schizophrenia.

Results showed that the cumulative risk for receiving a clinical diagnosis of schizophrenia after experiencing cannabis-induced psychosis was 21.4%. In addition, the conversion risk was 19.1% for those who had stimulant-induced psychosis, and it was 18.4% for those who had opioid-induced psychosis.

The number of patients who had first experienced each of these types of substance-induced psychoses was 276,273 and 419, respectively.

The largest substance-induced psychosis subgroup included those who had taken multiple substances (n = 1369). Their risk of conversion to schizophrenia was 21.5%. The second largest subgroup included those who had experienced alcohol-induced psychosis (n = 1038). However, their conversion risk was only 10.6%.

"Sedative, cocaine, hallucinogen, tobacco, and solvent induced psychosis groups were too small to allow meaningful interpretation," report the investigators.

Although the mean time to conversion to schizophrenia was approximately 12 years, the majority of diagnoses for the disorder occurred within the first 5 years after their substance-induced psychosis episode.

"Cannabis-induced psychosis or psychosis caused by multiple substances that requires hospital admission is more likely to be associated with later diagnosis schizophrenia than psychoses caused by other substances," write the researchers.

However, "conversion rates are still significant with other substance-induced psychoses and increased attention to this group of disorders is likely to pay dividends in the search for the causes of schizophrenia," they add.

Important, but Not Surprising

"This is an important study done in a large group of patients," session moderator Peter W. Woodruff, PhD, FRCPsych, professor of academic clinical psychiatry at the University of Sheffield, England, told Medscape Medical News after the presentation.

"It's clearly an important clinical problem in regards to patients taking cannabis, which is also a major risk factor for developing schizophrenia," added Dr. Woodruff, who was not involved with this research.

However, he noted that it was "not surprising," given that any substance that affects a person's state of mind can alter neurotransmitters in the brain.

"If nonprescribed drugs, or substances, are taken for nontherapeutic reasons by people who are otherwise healthy, these substances can cause some form of destabilization of the healthy dynamics and alter healthy function in susceptible people — and can increase risk of their developing serious mental illness."

International Congress of the Royal College of Psychiatrists (RCPsych) 2013. Presented July 5, 2013.