People at clinical high risk (CHR) of developing psychotic disorders such as schizophrenia who use tobacco and cannabis together are nearly three times more likely to develop psychosis, according to a new study published in Nature Mental Health.
Researchers from Vanderbilt Health analysed data involving more than 1,000 participants from the North American Prodrome Longitudinal Study, a multisite psychotic disorder study conducted across North America that collected data on CHR individuals.
Clinical data from CHR people who use tobacco and cannabis concurrently were compared to that of people who use both substances together and were not considered to be at risk of developing psychosis-based disorders.
The study found that CHR people co-using cannabis and tobacco were three times more likely to develop a psychosis-based disorder than those not at risk.
“The prevalence of cannabis and tobacco use, known as ‘co-use,’ has been rising in the general population for the past several decades, while exclusive tobacco use has declined and exclusive cannabis use has been on the rise,” said lead researcher Heather Ward. “However, little is known about cannabis and tobacco co-use in adolescents at risk for psychosis.”
The team looked for patterns of use of tobacco only, cannabis only, co-use, and no use of either substance in participants over two years between 2009 and 2012. The cohort comprised 734 CHR individuals, and 278 participants in the control group who were considered to be healthy.
Researchers found that the higher the frequency of tobacco and cannabis co-use, the higher the risk of developing a psychosis-based disorder became, and researchers found this risk to be present in both the CHR group and the control group.
The researchers did not find, however, that CHR individuals who co-used cannabis and tobacco experienced more severe symptoms of psychosis than those who didn’t co-use the substances.
The study did expose that those who use cannabis heavily (more than three times a week), with light tobacco use, were more than three times more likely to develop symptoms of psychosis than those who had used neither substance.
“We found that cannabis and tobacco co-use was associated with a nearly threefold increased risk of developing psychosis in people who were already at risk. There is evidence to suggest that using tobacco and cannabis together may have synergistic effects on the brain,” said Ward.
Previous research has shown that people living with psychosis and who smoke tobacco live on average 20 years fewer than those without psychosis and who don’t use tobacco. Similarly, it is estimated that 25-50% of those experiencing their first psychotic episode use cannabis.
When tobacco and cannabis are used together, tetrahydrocannabinol (THC) absorption increases in the body. Researchers were unsure if this was behind CHR individuals being more likely to develop symptoms of psychosis, or whether CHR individuals simply have an underlying predisposition to co-use both substances.
“Smoking tobacco and cannabis together increases absorption of THC, the psychoactive component of cannabis. It is possible that co-use itself is contributing to the development of psychosis. However, it is also possible that the people who are going to develop psychosis anyway have an underlying predisposition to using both cannabis and tobacco,” said Ward.
“People with psychosis are much more likely to use cannabis and tobacco than the general population. Because of their heavy cannabis and tobacco use, people with psychosis are also disproportionately affected by the negative consequences of cannabis and tobacco use,” the study said.
“Likewise, meta-analyses have concluded that, although cannabis is neither a necessary nor sufficient cause of psychosis, it probably contributes to psychosis risk in predisposed individuals.”
Ward added that stopping cannabis and tobacco co-use may reduce the risk of developing psychosis, though further research is needed to confirm this.
