Nearly triple the risk of developing psychosis when used together
A two-year study of more than a thousand young people at clinical risk for psychosis has found that combining cannabis and tobacco use nearly triples the likelihood of developing a full psychotic disorder — a finding that places a familiar and growing behavioral pattern inside a far more consequential story about vulnerability, neurology, and the compounding weight of risk. Led by psychiatrist Heather Ward at Vanderbilt Health and published in Nature Mental Health, the research suggests that the two substances together may act on the brain in ways neither does alone, accelerating a descent that might otherwise have been averted. For a population already carrying shortened life expectancies and heightened psychiatric burden, the question of whether cessation can reverse this trajectory is now the urgent one.
- Young people already showing early warning signs of psychosis who use cannabis and tobacco together face nearly three times the risk of developing a full psychotic disorder compared to those who use neither substance.
- The danger is not simply additive — tobacco appears to increase THC absorption when the substances are smoked together, potentially creating a neurological synergy that pushes vulnerable brains past a critical threshold.
- Co-use is rising even as tobacco-only use declines, and it remains poorly understood in high-risk populations, leaving clinicians without clear guidance at the moment the need is sharpest.
- People already diagnosed with psychosis use cannabis at rates of 25 to 50 percent and face 20-year reductions in life expectancy from tobacco — meaning the harms of co-use fall hardest on those least equipped to absorb them.
- The research team is now designing intervention studies to test whether helping high-risk individuals quit both substances can actually prevent psychosis from developing — a question the current data raises but cannot yet answer.
Researchers at Vanderbilt Health have identified a troubling convergence: when young people already showing early warning signs of psychosis use cannabis and tobacco together, their likelihood of developing a full psychotic disorder nearly triples. The finding emerges from a two-year study of over 1,000 participants in the North American Prodrome Longitudinal Study, which follows adolescents and young adults experiencing mild psychiatric symptoms that have not yet crystallized into schizophrenia or other psychotic disorders. Psychiatrist Heather Ward led the work, which was published in Nature Mental Health.
The study tracked 734 clinically high-risk individuals and 278 healthy controls, categorizing participants by substance use patterns over two years. A counterintuitive finding emerged: using cannabis or tobacco alone was associated with anxiety, depression, and early psychotic experiences, but short-term psychiatric symptoms did not worsen dramatically when people used both. The real damage unfolded over time — those who used cannabis heavily alongside tobacco were nearly three times more likely to convert to psychosis than those who used neither.
The mechanism appears biological. When smoked together, tobacco increases the absorption of THC, potentially creating a synergistic effect on the brain that accelerates psychotic development in those already predisposed. This matters because co-use is becoming more common even as tobacco-only use declines, and it remains poorly understood in vulnerable populations.
The human cost is severe. Among those already diagnosed with psychosis, cannabis use correlates with more intense symptoms, poor medication response, and repeated hospitalizations. Tobacco use in this population is linked to a 20-year reduction in life expectancy. People with psychosis are far more likely to use both substances than the general population, compounding an already devastating burden.
Ward stressed that clinicians and patients alike need to recognize co-use as a distinct and specific risk factor — not simply the sum of two individual harms. Whether helping high-risk individuals quit both substances can actually prevent psychosis remains the open question, and the next phase of research is designed to answer it. For now, the evidence is clear enough: in young people already walking close to the edge of psychotic illness, combining these two substances measurably narrows the distance to the cliff.
Researchers at Vanderbilt Health have documented a troubling pattern: when young people already at risk for psychosis use cannabis and tobacco together, their chances of developing a full psychotic disorder nearly triple. The finding comes from a two-year study of over 1,000 participants in the North American Prodrome Longitudinal Study, a research program that follows adolescents and young adults showing early warning signs of psychosis—mild symptoms that haven't yet crystallized into schizophrenia or other psychotic disorders. The work, led by psychiatrist Heather Ward, was published in Nature Mental Health and presented at the Society of Biological Psychiatry Annual Meeting.
The study tracked 734 people at clinical high risk for psychosis and 278 healthy controls, documenting their substance use patterns over two years. Researchers categorized participants by what they used: tobacco alone, cannabis alone, both substances together, other drugs, or nothing. The results revealed something unexpected. While using either cannabis or tobacco by itself was linked to anxiety, depression, and early psychotic experiences, the short-term psychiatric symptoms didn't worsen dramatically when people used both. The real damage appeared over time. Those who used cannabis heavily and tobacco lightly were almost three times more likely to develop psychosis than those who used neither substance.
This matters because the pattern of combined use is becoming more common. Tobacco-only use has declined in recent decades while cannabis use has risen, but the overlap between the two—what researchers call co-use—has been climbing and remains poorly understood, especially in vulnerable populations. Ward and her team define co-use as using the substances at the same time, on the same occasion, or within a timeframe where their effects overlap. The mechanism appears biological. When tobacco and cannabis are smoked together, tobacco increases the absorption of THC, the psychoactive component of cannabis, potentially creating a synergistic effect on the brain that pushes vulnerable individuals toward psychosis.
The human stakes are severe. Among people already diagnosed with psychosis, roughly 25 to 50 percent use cannabis. That use correlates with more intense psychotic symptoms, poor response to psychiatric medications, and repeated hospitalizations. Tobacco use in this population carries its own catastrophic toll: a 20-year reduction in life expectancy compared to the general population, driven by cardiovascular disease, heart attack, stroke, and lung cancer. People with psychosis are far more likely to use both substances than the general population, meaning they bear a disproportionate burden of the harms both drugs cause.
Ward emphasized that the findings have immediate clinical implications. Both patients and clinicians need to understand that co-use is a specific risk factor for psychosis, distinct from the risks of either substance alone. Stopping cannabis and tobacco use may improve existing mental health symptoms, and it's possible that preventing co-use could reduce the risk of developing a psychotic disorder in the first place. The next phase of research will test whether this is true—whether interventions that help people quit both substances actually reduce the likelihood of psychosis developing in high-risk groups.
What remains unclear is causation. The threefold increase in psychosis risk could mean that co-use itself triggers psychosis in vulnerable brains. Or it could mean that people with an underlying predisposition to psychosis are simply more likely to use both substances. Ward's team is designed to answer that question, but for now, the evidence is clear enough: in young people already walking the edge of psychotic illness, combining cannabis and tobacco is a measurable step toward the cliff.
Notable Quotes
Cannabis and tobacco co-use was associated with a nearly threefold increased risk of developing psychosis in people who were already at risk.— Heather Ward, MD, Vanderbilt Health
Stopping use of cannabis and tobacco may improve mental health symptoms, and it is possible that stopping cannabis and tobacco co-use could reduce risk of developing psychosis in the first place.— Heather Ward, MD
The Hearth Conversation Another angle on the story
Why does combining these two substances seem worse than using either one alone?
The tobacco appears to act as an amplifier. When you smoke them together, the tobacco increases how much THC your body absorbs. It's not just additive—it's synergistic. The brain gets hit with a larger dose of the psychoactive component, and in people already at risk, that seems to tip them over.
But the study found that short-term symptoms didn't get worse with co-use. How do you square that?
That's the unsettling part. In the moment, people using both substances didn't report worse anxiety or depression than those using just one. But over the two-year period, the ones using both were three times more likely to actually develop psychosis. It's as if the damage accumulates silently.
Is it possible these people were already destined to develop psychosis and just happened to use both substances?
That's the honest answer we don't have yet. The study shows correlation, not proof of cause. People with an underlying vulnerability to psychosis might naturally gravitate toward both drugs. The next studies need to test whether stopping co-use actually prevents psychosis from developing.
What should a clinician do with this information right now?
Take it seriously as a warning sign. If you're seeing a young person at clinical high risk for psychosis and they're using both cannabis and tobacco, that combination is now a documented risk factor. It's worth addressing directly, not as a side issue.
The life expectancy gap is staggering—20 years lost to tobacco in people with psychosis.
Yes. And that's just the tobacco piece. Add in the cannabis-related hospitalizations, treatment resistance, and worsening symptoms, and you're looking at a population bearing an enormous burden. They're already vulnerable, and these substances compound that vulnerability in ways we're only now beginning to measure.