A Cannabis-Tobacco Combination May Triple Your Schizophrenia Risk

Roughly one in three teenagers who use cannabis heavily will also use tobacco at the same time. That pattern isn’t just a lifestyle quirk – new research suggests it may be one of the most significant schizophrenia risk factors scientists have identified in years, and the mechanism behind it is more unsettling than most people realize.

For decades, researchers studied cannabis and tobacco in isolation when assessing their effects on mental health. Cannabis has a known association with psychosis. Tobacco had its own complicated relationship with psychiatric conditions. What nobody had fully examined was what happens when the two are used together – regularly, repeatedly, and in the developing brain. The answer, it turns out, changes the math considerably.

The concern intensifies because of timing. Both substances are being used at younger ages, cannabis potency has climbed sharply over the past two decades, and co-use, the practice of using both substances simultaneously or within close time frames, has been rising in the general population even as exclusive tobacco use has fallen. Researchers are now racing to understand what that shift means for the population of young people most at biological risk for psychotic illness.

A Threefold Risk: What the New Research Found

A multisite study published in Nature Mental Health found that using cannabis and tobacco together increases the risk of developing psychotic disorders like schizophrenia among those considered high risk. The study, led by Dr. Heather Ward, assistant professor of Psychiatry and Behavioral Sciences and director of Neuromodulation Research at Vanderbilt Health, is one of the most detailed examinations of co-use and psychosis risk to date.

Substance use patterns – tobacco only, cannabis only, co-use, and no substance use – were assessed over a two-year period in 734 individuals at clinical high risk for psychosis and 278 healthy controls. These individuals often experience mild or early symptoms but have not yet developed a full psychotic disorder. That distinction matters: this is a population already standing at the edge of a psychiatric threshold.

Those who used cannabis heavily and tobacco lightly were almost three times more likely to develop psychosis compared to those who used neither substance. Researchers defined co-use as “using substances at the same time, on the same occasion, or within a defined time frame where their effects may overlap.” The finding held even after accounting for other variables, pointing to something specific about the combination rather than just the individual substances.

There is evidence to suggest that using tobacco and cannabis together may have synergistic effects on the brain. Smoking tobacco and cannabis together increases absorption of THC, the psychoactive component of cannabis. That pharmacological interaction – where nicotine essentially amplifies THC delivery to the brain – may explain why the combined risk is so much higher than either substance alone.

What Happens in the Brain

The brain science behind this combination involves two distinct but interacting systems. Using a specialized form of magnetic resonance imaging, researchers found that individuals who used cannabis heavily over time had increased dopamine-related signals in a midbrain region previously associated with psychotic symptoms. Brain imaging was conducted using a technique known as neuromelanin-sensitive MRI, which indirectly reflects dopamine activity in the substantia nigra and ventral tegmental area – regions critical for motivation, reward, and the processing of salient stimuli.

According to researchers at London Health Sciences Centre Research Institute, “excess levels of dopamine can disrupt normal brain processes and may increase the risk of psychosis, particularly in individuals who are already vulnerable.” In schizophrenia, this dopamine dysregulation is a central feature – and cannabis appears to push the system in that exact direction.

Tobacco adds a separate layer of interference. Co-users of cannabis and tobacco show measurable differences in brain chemistry, including elevated levels of an enzyme called FAAH (fatty acid amide hydrolase), which breaks down anandamide – a naturally occurring compound sometimes called the brain’s “bliss molecule.” Reduced anandamide levels have been linked to higher anxiety and depression. When both substances are used together, the endocannabinoid system – the brain’s internal regulatory network for mood, stress, and perception – is being hit from two directions at once.

Dr. Ward noted that both patients and clinicians need to know that cannabis and tobacco co-use is a risk factor for psychosis, and that stopping use of both substances may improve mental health symptoms and could potentially reduce the risk of developing psychosis in the first place.

The Adolescent Window

A separate but equally striking line of research focuses specifically on teenagers. A large study published in JAMA Health Forum found that adolescents who use cannabis face a significantly higher risk of developing serious psychiatric disorders by young adulthood. The longitudinal study followed 463,396 adolescents ages 13 to 17 through age 26 and found that past-year cannabis use was associated with a doubled risk of incident psychotic disorders and bipolar disorder, along with significantly higher rates of depression and anxiety.

The research tracked 463,396 adolescents aged 13 to 17 through age 26 using electronic health record data from Kaiser Permanente Northern California. Lead author Kelly Young-Wolff, Ph.D., a senior research scientist at the Kaiser Permanente Division of Research, noted that “even after accounting for prior mental health conditions and other substance use, adolescents who reported cannabis use had a substantially higher risk of developing psychiatric disorders – particularly psychotic and bipolar disorders.”

The timing of cannabis use relative to psychiatric diagnosis is one of the study’s most clinically relevant details. Researchers found that self-report of past-year cannabis use typically occurred an average of 1.7 to 2.3 years before a psychiatric diagnosis. That temporal sequence – cannabis use consistently preceding the disorder – is consistent with a causal direction, though researchers emphasize the study cannot definitively prove causation. It’s possible some adolescents begin using cannabis in response to emerging symptoms, but the time gap makes that a less complete explanation.

Results showed that cannabis use was associated with a doubled risk of psychotic and bipolar disorders, a 34% increased risk of depression, and a 24% increased risk of anxiety. Those figures held across the socioeconomic, gender, and health-status diversity of a massive, real-world health system population – not a self-selected survey group.

Potency Is Part of the Problem

One factor that makes today’s findings harder to compare with older research is the dramatic increase in THC concentration in commercially available cannabis. High-THC options now dominate many cannabis markets. Average THC levels in California cannabis flower now exceed 20%, far higher than in previous decades, while concentrates can exceed 95% THC, according to data from the Public Health Institute. THC concentration in vaping cartridges can reach 80 to 90%, compared to smoked marijuana which typically contains 10 to 20% THC.

That potency shift matters because the psychosis risk associated with cannabis is dose-related. Lifetime cannabis use is associated with a 1.4-fold increase in the risk of developing psychotic illness, with cannabis dependence conferring a 3.4-fold increase. As products become more concentrated, users who believe they are consuming moderate amounts may actually be receiving doses far above what older risk estimates were based on.

Researcher Dr. Lynn Silver of the Public Health Institute stated that “as cannabis becomes more potent and aggressively marketed, this study indicates that adolescent cannabis use is associated with double the risk of incident psychotic and bipolar disorders, two of the most serious mental health conditions.”

Who Is Most at Risk

Understanding schizophrenia risk factors requires recognizing that not everyone who uses cannabis and tobacco will develop psychosis. The growing body of evidence points to cannabis use during adolescence as carrying potentially detrimental, long-term health effects – but vulnerability varies. Genetics, age of first use, frequency of use, and whether co-use is involved all appear to shape individual risk.

Up to 62% of people with schizophrenia use tobacco – a prevalence three times that of the general population. Across schizophrenia spectrum disorders, cannabis use disorders are present in 26.2% of individuals, with prevalence rising to 36% among those with first-episode psychosis. Those numbers suggest that substance use and psychotic illness are deeply entangled – though researchers continue to debate the direction of that relationship.

People at clinical high risk for psychosis – those who experience attenuated psychotic symptoms, brief episodes, or have a close family member with schizophrenia – face a particularly narrow margin. Given the consequences of cannabis and tobacco use, understanding how their co-use affects psychiatric symptoms and the development of psychosis is critical. For that group, adding tobacco to cannabis use may be the variable that tips an already-unstable system toward a full psychotic break.

For parents and clinicians, one concrete warning sign is the trajectory of use. The CDC notes that people who use cannabis are more likely to develop psychosis and longer-lasting disorders, adding that “the association between cannabis and schizophrenia is stronger in people who start using cannabis at an earlier age and use cannabis more frequently.” Frequency, potency, and co-use are the three levers most clearly linked to elevated psychiatric risk.

You can read more about the rising rates of cannabis-related mental health presentations in this 2026 report on cannabis-induced psychosis.

What to Do Now

The Vanderbilt-led study cannot prove that cannabis and tobacco directly cause psychosis – other genetic, environmental, or mental health factors also contribute. But the magnitude and consistency of the findings across two major 2026 studies is difficult to set aside, especially for anyone with a personal or family history of psychotic illness.

Stopping use of cannabis and tobacco may improve mental health symptoms, and Dr. Ward’s team has called for further research to test whether cessation of cannabis and tobacco co-use can reduce the risk of developing a psychotic disorder in people who are already considered high risk. That research is underway, but the available evidence already supports one practical conclusion: the combination is riskier than either substance alone, and for people at elevated risk, avoiding both is a reasonable and evidence-grounded choice.

For teenagers specifically, the data from nearly half a million Kaiser Permanente patients makes a strong case for early conversations about cannabis – not as a moral issue, but as a neurodevelopmental one. The adolescent brain is still developing its dopamine regulation and prefrontal control systems well into the mid-twenties. Introducing high-potency THC during that window, particularly alongside nicotine, may be altering trajectories that cannot easily be corrected later.

If you or someone in your life is at clinical high risk for psychosis, or has a first-degree relative with schizophrenia, the current evidence justifies a direct conversation with a psychiatrist about substance use patterns. The conversation doesn’t need to wait for a crisis.

Disclaimer: The information provided here is for educational and informational purposes only and is not a substitute for professional psychological, psychiatric, or mental health advice, diagnosis, or treatment. Always seek the guidance of a licensed mental health professional, therapist, psychologist, or psychiatrist with any questions or concerns about your emotional well-being or mental health conditions. Never ignore professional advice or delay seeking support because of something you have read here.

AI Disclaimer: This article was created with the assistance of AI tools and reviewed by a human editor.

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