People with psychosis who used cannabis heavily before getting sick had higher genetic risk for schizophrenia
Patients who used cannabis weekly to daily before their first psychotic episode had significantly higher polygenic risk scores for schizophrenia, with the strongest effect in those who started before age 18.
Quick Facts
What This Study Found
Researchers assigned schizophrenia polygenic risk scores to 381 schizophrenia spectrum patients, 220 bipolar disorder spectrum patients, and 415 healthy controls. The polygenic risk score reflects how many common genetic variants associated with schizophrenia each person carries.
Patients who had used cannabis weekly to daily before their first illness episode had the highest genetic risk scores for schizophrenia, regardless of whether they were diagnosed with schizophrenia or bipolar disorder. The difference was statistically significant.
The effect was strongest among those who began heavy cannabis use before age 18. These early-onset frequent users had the largest difference in genetic risk compared to patients who never or rarely used cannabis, with a moderate effect size of 0.42.
Key Numbers
381 schizophrenia spectrum patients, 220 bipolar spectrum patients, 415 healthy controls. Weekly-to-daily cannabis users before illness had highest polygenic risk scores (p = 0.02, Cohen's d = 0.33). Early-onset frequent users (before age 18) versus non-users: p = 0.003, Cohen's d = 0.42. Reference sample for risk scores: 81,535 participants from PGC.
How They Did This
This was an observational genetic study using polygenic risk scores calculated from the Psychiatric Genomics Consortium schizophrenia case-control study (N = 81,535). An independent sample of 381 schizophrenia spectrum cases, 220 bipolar spectrum cases, and 415 healthy controls was genotyped and scored. Cannabis use history before illness onset was collected and participants were grouped by frequency of use.
Why This Research Matters
This finding helps untangle the complex relationship between cannabis and psychosis. Rather than cannabis simply "causing" psychosis, the data suggest that individuals with higher genetic loading for schizophrenia may be more likely to use cannabis heavily before becoming ill. This does not rule out cannabis as an additional environmental trigger, but it suggests the relationship involves both genetic vulnerability and cannabis exposure.
The Bigger Picture
This study contributes to the gene-environment interaction model of psychosis. It suggests that the link between cannabis and psychosis is not a simple one-way street but involves a complex interplay where genetic vulnerability both increases psychosis risk and may increase the likelihood of heavy cannabis use. Understanding this interaction is crucial for identifying who is truly at risk.
What This Study Doesn't Tell Us
The study cannot determine causality or direction of effect. It is possible that the same genes that increase schizophrenia risk also increase cannabis use propensity. Cannabis use history was based on retrospective self-report, which may be inaccurate. The study focused on common genetic variants and did not account for rare mutations or environmental factors beyond cannabis.
Questions This Raises
- ?Do the same genetic variants that increase schizophrenia risk also drive cannabis-seeking behavior?
- ?Could polygenic risk scores eventually be used to identify individuals who should avoid cannabis?
- ?Does cannabis use in genetically high-risk individuals accelerate illness onset rather than cause it?
Trust & Context
- Key Stat:
- Early heavy cannabis users had the highest schizophrenia genetic risk scores (Cohen's d = 0.42)
- Evidence Grade:
- This is a well-powered genetic study with a large reference sample, but the observational design limits causal conclusions.
- Study Age:
- Published in 2018, using genetic data from the Psychiatric Genomics Consortium.
- Original Title:
- Psychotic patients who used cannabis frequently before illness onset have higher genetic predisposition to schizophrenia than those who did not.
- Published In:
- Psychological medicine, 48(1), 43-49 (2018)
- Authors:
- Aas, M, Melle, I, Bettella, F, Djurovic, S, Le Hellard, S, Bjella, T, Ringen, P A, Lagerberg, T V, Smeland, O B, Agartz, I, Andreassen, O A, Tesli, M
- Database ID:
- RTHC-01561
Evidence Hierarchy
Watches what happens naturally without intervening.
What do these levels mean? →Frequently Asked Questions
Does this mean cannabis use is genetic?
Not exactly. The study found that people who developed psychosis and had used cannabis heavily before getting sick tended to carry more genetic variants associated with schizophrenia. This could mean genetic vulnerability increases both psychosis risk and the tendency toward heavy cannabis use.
Can genetic testing predict who should avoid cannabis?
Not yet with clinical reliability. Polygenic risk scores capture statistical tendencies across populations, but they are not precise enough to make individual predictions. Research in this area is ongoing.
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Cite This Study
https://rethinkthc.com/research/RTHC-01561APA
Aas, M; Melle, I; Bettella, F; Djurovic, S; Le Hellard, S; Bjella, T; Ringen, P A; Lagerberg, T V; Smeland, O B; Agartz, I; Andreassen, O A; Tesli, M. (2018). Psychotic patients who used cannabis frequently before illness onset have higher genetic predisposition to schizophrenia than those who did not.. Psychological medicine, 48(1), 43-49. https://doi.org/10.1017/S0033291717001209
MLA
Aas, M, et al. "Psychotic patients who used cannabis frequently before illness onset have higher genetic predisposition to schizophrenia than those who did not.." Psychological medicine, 2018. https://doi.org/10.1017/S0033291717001209
RethinkTHC
RethinkTHC Research Database. "Psychotic patients who used cannabis frequently before illne..." RTHC-01561. Retrieved from https://rethinkthc.com/research/aas-2018-psychotic-patients-who-used
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.