Does Cannabis Use Influence Schizophrenia? Study Sheds New Light

Cannabis use and schizophrenia are linked, but interpreting that link requires care. An observational study cannot settle causation by itself. It also cannot establish that cannabis has no causal role. The practical question is how the evidence should inform decisions about cannabis and prompt recognition of mental health symptoms.

What the Ontario Study Examined

A 2025 study in JAMA Network Open followed approximately 13.6 million Ontario residents aged 14–65 without prior schizophrenia during 2006–2022. It identified cannabis use disorder through emergency department or hospital records, rather than measuring all cannabis use.

The adjusted population-attributable risk fraction rose from 3.7% before liberalization to 10.3% after nonmedical legalization. Overall schizophrenia incidence remained stable. The fraction increased steadily, without acceleration after the policy changes.

What Those Percentages Mean

The fraction estimates the share of schizophrenia cases potentially preventable by eliminating the exposure, assuming the association is causal and adequately adjusted for other factors. It does not mean 10.3% of cannabis users develop schizophrenia. The highest subgroup estimate, 18.9% among males aged 19–24, is also a population fraction, not an individual probability.

Unmeasured factors, including family history and genetics, limit causal interpretation. Hospital records also miss cannabis use disorder managed elsewhere. The study does not demonstrate that legalization alone caused the increase, or that cannabis merely worsens already diagnosed schizophrenia: participants had no prior schizophrenia diagnosis.

How This Fits the Broader Evidence

The CDC describes a higher likelihood of psychosis and schizophrenia among people who use cannabis. The association is stronger with earlier initiation and more frequent use. Cannabis can also cause disorientation, anxiety, and paranoia.

These findings warrant attention without implying that every person who uses cannabis will develop schizophrenia. Equally, uncertainty about the contribution of different risk factors should not be presented as proof that cannabis is harmless to mental health.

Psychosis and Schizophrenia Are Different

According to the National Institute of Mental Health, psychosis involves disrupted thoughts or perceptions and difficulty distinguishing reality. Symptoms can include hallucinations, delusions, and confused speech. A psychotic episode does not automatically mean schizophrenia.

Psychosis can occur with several mental illnesses, substance use, sleep deprivation, or other medical causes. Assessment by a qualified clinician matters because the explanation and treatment can differ. Genetic vulnerability, brain development, and stress or trauma may also contribute.

When to Seek Help

Persistent or worsening paranoia, hearing or seeing things others do not, or difficulty telling what is real deserve prompt professional assessment. Tell the clinician about cannabis and other substances, medications, sleep changes, and when symptoms began.

Early treatment can improve recovery. Coordinated specialty care combines several forms of support for early psychosis. In the United States, call or text 988 for a mental health crisis; call 911 for a life-threatening emergency.