Vanderbilt Health researchers have confirmed that co-using cannabis and tobacco drastically elevates the risk of developing psychotic disorders in vulnerable populations. Published in Nature Mental Health on May 12, the study highlights how this growing, understudied trend directly accelerates psychiatric decline in at-risk youth.
Led by Dr. Heather Ward, the research team analyzed data from over 1,000 participants in the North American Prodrome Longitudinal Study. The cohort consisted of individuals at “clinical high risk” who exhibited early, mild symptoms but had not yet developed full psychosis.
While exclusive tobacco use has declined in the general population, exclusive cannabis use and the simultaneous “co-use” of both substances have risen over the past several decades. Dr. Ward noted that people with psychosis are much more likely to use these substances and are disproportionately affected by their negative consequences.
The devastating impacts of isolated substance use in psychiatric patients are well-documented:
- Tobacco Impact: Associated with a 20-year decreased life expectancy due to cardiovascular disease, stroke, and lung cancer.
- Cannabis Impact: Used by 25% to 50% of people in their first psychotic episode, leading to more severe symptoms, psychiatric hospitalizations, and poor treatment response.
To understand the combined impact, researchers tracked 734 high-risk individuals and 278 healthy controls over a two-year period. While regular use of either substance was linked to anxiety and early psychotic experiences, the most significant difference emerged over time for co-users.
The study found that individuals who heavily used cannabis while lightly using tobacco were almost three times more likely to transition into full psychosis compared to those who abstained from both.
“Smoking tobacco and cannabis together increases absorption of THC, the psychoactive component of cannabis,” Dr. Ward explained. She suggested that this combination may have synergistic, damaging effects on the brain, though it is also possible that those predisposed to psychosis are naturally drawn to co-use.
Moving forward, researchers emphasize that both patients and clinicians must recognize cannabis and tobacco co-use as a critical risk factor. The next phase of research will focus on whether halting the use of these substances can actively prevent the onset of psychotic disorders in vulnerable groups.