Cannabis and tobacco use often go hand in hand. For young people who already show early signs of psychosis, that habit may carry more weight than anyone assumed. Two linked studies from the same North American cohort point to a higher chance of developing a psychotic disorder, and weaker results on thinking tasks, among those who use both.
Daniel Bello and colleagues at Vanderbilt University Medical Center posted the first study on medRxiv in September 2025. Peer review hasn’t happened yet.
A second paper, in Biological Psychiatry: Cognitive Neuroscience and Neuroimaging in August 2026, checks whether the pattern also shows up in memory, attention and learning.
Why Cannabis and Tobacco Use Matters for Young People at Risk
Psychosis and substance use overlap heavily, and the numbers show it. Up to 60 per cent of people with schizophrenia use tobacco, three times the general population rate. Cannabis use disorder affects 43 per cent of people with schizophrenia, against 15 per cent in the wider population. Tobacco is the leading preventable cause of early death in schizophrenia, and life expectancy runs about 20 years shorter.
The habits are converging, too. Among 18 to 25 year olds in the United States, one in five daily cigarette smokers also uses cannabis every day. Daily cannabis use among cigarette smokers rose from 4.9 per cent in 2002 to 9 per cent in 2014, according to the authors.
Still, few studies had asked what cannabis and tobacco use together means for people at clinical high risk of psychosis. That is the stage before illness fully arrives, when faint symptoms, low mood and social strain begin to show. Timing matters here. Substance use usually starts in adolescence, before psychosis does, and roughly 77 per cent of people report trying tobacco before their first psychotic symptoms.
Who Took Part in the Research
Both papers use NAPLS2, the North American Prodrome Longitudinal Study 2, which tracked young people across eight sites. The preprint analysed 1,014 people: 734 at clinical high risk and 280 healthy controls. Everyone reported their substance use over the past 30 days, and researchers then followed them for two years. In that time, 83 of the 734 at risk participants developed a psychotic disorder.
The team split the at risk group by use. Some used cannabis only, some tobacco only, some both, some other substances such as alcohol, and some nothing at all. In the cognition paper, 97 people used both, 78 used tobacco only and 79 used cannabis only. Average ages across groups ranged from about 17 to 21.
Combined Use of Cannabis and Tobacco and the Psychosis Hazard
The headline finding concerns risk. People who used both substances had a hazard ratio of 2.53 for developing psychosis, compared with people who used neither. Put simply, their risk more than doubled over follow up. The 95 per cent confidence interval ran from 1.44 to 4.45.
Heavy use lifted the number further. At that level, cannabis and tobacco use together produced a hazard ratio of 3.63, a more than threefold increase. By contrast, cannabis alone (1.54) and tobacco alone (1.69) did not reach statistical significance as separate groups.
Why might that be? The authors give two possible explanations. Using both could amplify what either does to the brain’s dopamine systems. Or the pairing may simply mark someone whose risk was already higher. The data can’t tell these apart.
Cannabis Appears to Carry the Heavier Weight
Cannabis held up when the team adjusted for tobacco. More frequent use carried a hazard ratio of 1.22. Tobacco’s link dropped to 1.15 and lost significance once cannabis entered the model. The authors call cannabis the primary culprit, although they point to earlier meta analyses that treat it as a contributor in predisposed people, not a sole cause.
Symptoms painted a muddier picture. Across the full sample, more frequent use of either substance went with worse psychosis, anxiety and depression scores. Within the at risk groups, though, baseline severity looked much the same. Neither substance showed any sign of easing symptoms.
What the Second Study Found About Thinking Skills
The Biological Psychiatry paper covered 1,012 people: 734 at risk and 278 healthy controls. Dr Heather Burrell Ward of Vanderbilt University Medical Center is the corresponding author. In this study, participants completed the MATRICS Consensus Cognitive Battery and an auditory continuous performance task.
Young people who used both substances scored below healthy controls on global cognition, working memory, verbal learning and attention. On the auditory task, the combined use group also showed weaker vigilance than either single substance group. The authors suggest those vigilance and working memory gaps might help explain the link to psychosis.
Reading the Cognitive Findings With Care
Not everything fits a simple story. Cannabis and tobacco use did not behave the same way on every measure. The cannabis only group matched healthy controls on global cognition. Their estimated IQ topped that of the combined use and tobacco groups, and heavier cannabis use went with better working memory. The authors suspect these young people may simply start out with stronger cognitive function.
Another oddity: at risk participants who used nothing scored lower on several measures and had the weakest social functioning of any at risk group. One explanation from the authors is that sociable young people meet substances more often, while those with severe negative symptoms step back from those settings.
None of this makes substance use protective. The study looked at one point in time, relied on self reported use over 30 days and couldn’t measure cannabis potency. Data came from 2009 to 2013, so potency has climbed since. Cause and direction remain open. As for nicotine, the authors doubt it worsens thinking. People with underlying cognitive difficulties may be drawn to it instead.
What This Means for Early Intervention
Clinicians should ask about both substances as a matter of routine, the authors say, because the pairing may flag a more severe cognitive risk profile than either alone. They add that reducing use in this group is warranted, since cutting back might delay or prevent psychosis for some people.
For families, schools and health services, the takeaway is simple. A conversation about cannabis and tobacco use costs little and can start early. However, asking about cannabis alone could miss half the picture. The researchers also want larger, longer studies that track people over time, measure use in finer detail and include newer high potency products and vaping.
Until that evidence arrives, the message holds. For young people already at risk of psychosis, cannabis and tobacco use deserves attention sooner rather than later.
Dalgarno Institute
Sources:
Cannabis and Tobacco Co-Use Predicts Psychosis in Clinical High Risk Cohorts
Cannabis and Tobacco Co-Use Is Associated With Impaired Neurocognitive Performance in Individuals at Clinical High Risk for Psychosis