Danish study of 3 million people found schizophrenia doubles the risk of developing cannabis abuse
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.
Quick Facts
What This Study 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.
Key Numbers
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 They Did This
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.
Why This Research 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 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.
What This Study Doesn't 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.
Questions This Raises
- ?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?
Trust & Context
- Key Stat:
- 2.5x higher risk of cannabis abuse after schizophrenia diagnosis
- Evidence Grade:
- Strong: national population register study of 3.1 million people with over 100 million person-years of follow-up and comprehensive confounding adjustment.
- Study Age:
- Published in 2019.
- Original Title:
- Schizophrenia is associated with increased risk of subsequent substance abuse diagnosis: A nation-wide population-based register study.
- Published In:
- Addiction (Abingdon, England), 114(12), 2217-2226 (2019)
- Database ID:
- RTHC-02227
Evidence Hierarchy
Enrolls participants and follows them forward in time.
What do these levels mean? →Frequently Asked 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.
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Cite This Study
https://rethinkthc.com/research/RTHC-02227APA
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
MLA
Petersen, Stine Mai, et al. "Schizophrenia is associated with increased risk of subsequent substance abuse diagnosis: A nation-wide population-based register study.." Addiction (Abingdon, 2019. https://doi.org/10.1111/add.14746
RethinkTHC
RethinkTHC Research Database. "Schizophrenia is associated with increased risk of subsequen..." RTHC-02227. Retrieved from https://rethinkthc.com/research/petersen-2019-schizophrenia-is-associated-with
Access the Original Study
Study data sourced from PubMed, a service of the U.S. National Library of Medicine, National Institutes of Health.
This study breakdown was produced by the RethinkTHC research team. We analyze and report published research findings without making health recommendations. All interpretations are based solely on the published abstract and study data.