The study involved more than 3,000 inpatients from Scotland who experienced substance-induced psychoses. The findings showed that those who suffered from psychosis resulting from cannabis, or polypharmacy (the use of multiple substances), that required hospitalization had the greatest increased risk of developing schizophrenia.
Conversion rates to schizophrenia were also high for those who had had episodes of stimulant- and opioid-induced psychoses.
Furthermore, the researchers found that the majority of patients who converted to schizophrenia did so within five 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,” said lead author David M. Semple, M.B.B.S., M.R.C.Psych., consultant psychiatrist from Hairmyres Hospital, East Kilbride, Glasgow, Scotland.
“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 percent, or about 1 in 4,” he said.
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 five years.”
For the study, researchers set out to investigate 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 looked at data from the Scottish Morbidity Record on 3,486 patients who were admitted to a psychiatric hospital between January 1997 and June 2012 for a first episode of substance-induced psychosis. These patients were then followed until July 2012 or until they experienced a first episode of schizophrenia.
The findings revealed that the cumulative risk for developing schizophrenia after experiencing cannabis-induced psychosis was 21.4 percent. The conversion risk was 19.1p ercent for those who had stimulant-induced psychosis, and 18.4 percent for those who had opioid-induced psychosis.
The risk of conversion to schizophrenia in those who had taken multiple substances was 21.5 percent. The conversion rate of alcohol-induced psychosis was only 10.6 percent.
“Sedative, cocaine, hallucinogen, tobacco, and solvent-induced psychosis groups were too small to allow meaningful interpretation,” investigators said.
Although the mean time to the development of schizophrenia was approximately 12 years, most diagnoses for the disorder occurred within the first five years after the 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 added.
“This is an important study done in a large group of patients,” said Peter W. Woodruff, Ph.D., F.R.C.Psych., professor of academic clinical psychiatry at the University of Sheffield, England. However, he noted that it was “not surprising,” since 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.”