Danish study of 3 million people found schizophrenia doubles the risk of developing cannabis abuse | Research

The takeaway

In a Danish population study of over 3 million people, a schizophrenia diagnosis was associated with a 3.7-fold increased risk of subsequent substance abuse, with cannabis showing the highest adjusted risk at 2.5 times that of the general population.

Psychiatrists, addiction specialists, and researchers studying the cannabis-psychosis relationship.

2.5x higher risk of cannabis abuse after schizophrenia diagnosis

What the researchers found

Among 3.1 million individuals followed for over 100 million person-years, 14,007 developed schizophrenia, and 2,885 were subsequently diagnosed with substance abuse. Schizophrenia was associated with increased risk of cannabis abuse (aHR 2.48), alcohol abuse (aHR 1.94), stimulant abuse (aHR 1.77), and other substance abuse (aHR 1.36). The association persisted 10-15 years after schizophrenia diagnosis.

Why it matters

Most research focuses on whether cannabis causes psychosis. This study flips the question: it shows that schizophrenia itself is a strong risk factor for developing substance abuse, particularly cannabis abuse. This has implications for treatment and monitoring.

The numbers in context

Cohort: 3,133,968 individuals. 103,212,328 person-years at risk. 14,007 developed schizophrenia. Overall substance abuse HR: 3.69 (95% CI 3.56-3.83). Cannabis-specific aHR: 2.48 (95% CI 2.34-2.64). Effect persisted 10-15 years (HR 2.50).

How the study worked

Prospective cohort study using Danish national registers covering all individuals born 1955-1999, followed from 1968-2013. Cox regression adjusted for calendar year, gender, urbanicity, co-abuse, other psychiatric diagnoses, parental substance abuse and psychiatric history, and socioeconomic position.

What this study cannot tell us

Observational register data; substance abuse diagnoses may undercount actual substance use. The one-year exclusion window (substance abuse diagnosed within a year of schizophrenia) was chosen to reduce reverse causation but is somewhat arbitrary.

How to read the evidence

Strong: national population register study of 3.1 million people with over 100 million person-years of follow-up and comprehensive confounding adjustment.

When this study was published

Published in 2019.

The bigger picture

The relationship between cannabis and psychosis is often framed as one-directional. This massive population study demonstrates that schizophrenia also drives cannabis use, suggesting a bidirectional relationship that treatment programs need to address.

Questions still open

Does treating schizophrenia more effectively reduce subsequent substance abuse? Are there subtypes of schizophrenia with higher substance abuse risk? Could this bidirectional relationship create a vicious cycle that accelerates both conditions?

Common questions

Does this mean schizophrenia causes cannabis use?

The study found a strong association between schizophrenia diagnosis and subsequent substance abuse, but observational data cannot definitively prove causation. Self-medication, shared genetic vulnerability, and social factors may all contribute.

Why is cannabis the top substance risk for people with schizophrenia?

The study did not explain why, but possible reasons include self-medication for symptoms, shared genetic risk factors, or the accessibility and perceived harmlessness of cannabis compared to other substances.

Read the original research

Schizophrenia is associated with increased risk of subsequent substance abuse diagnosis: A nation-wide population-based register study.

Addiction (Abingdon, England), 114(12), 2217-2226

View on PubMed ↗Publisher’s paper ↗

Citation

Petersen, Stine Mai; Toftdahl, Nanna Gilliam; Nordentoft, Merete; Hjorthøj, Carsten. (2019). Schizophrenia is associated with increased risk of subsequent substance abuse diagnosis: A nation-wide population-based register study.. Addiction (Abingdon, England), 114(12), 2217-2226. https://doi.org/10.1111/add.14746