In a double-blind crossover trial, pre-treatment with 1000mg CBD before cannabis inhalation worsened memory impairment and psychotic symptoms in patients with schizophrenia, contradicting the protective hypothesis.
Psychiatrists, psychosis researchers, harm reduction advocates
CBD pre-treatment worsened both memory (p=0.001) and psychotic symptoms (p=0.01) after cannabis use in schizophrenia
What the researchers found
CBD pre-treatment resulted in worse delayed verbal recall (3.5 vs 4.8 words, p=0.001) and greater increase in positive psychotic symptoms (5.0 vs 2.9 PANSS-P increase, p=0.01) compared to placebo pre-treatment; CBD did not alter THC or 11-OH-THC plasma levels.
Why it matters
This directly challenges the popular assumption that CBD protects against THC's harmful effects, particularly in the most vulnerable population.
The numbers in context
30 participants; CBD 1000mg vs placebo; delayed recall 3.5 vs 4.8 words (MD -1.3, 95% CI: -2.0 to -0.6); PANSS-P increase 5.0 vs 2.9 (MD 2.2, 95% CI: 0.6 to 3.7); no difference in THC plasma levels.
How the study worked
Double-blind, randomized, placebo-controlled, crossover trial; 30 participants with schizophrenia/schizoaffective disorder plus cannabis use disorder; oral CBD 1000mg or placebo 3 hours before vaporized cannabis (THC 20-60mg).
What this study cannot tell us
Small sample (n=30); single CBD dose; acute effects only; participants had comorbid cannabis use disorder; may not generalize to healthy volunteers.
How to read the evidence
Double-blind crossover RCT provides strong within-subject evidence, though small sample and single acute session limit generalizability.
When this study was published
Published 2025
The bigger picture
The idea that CBD in cannabis products might protect against THC-related psychosis has influenced product marketing and harm reduction messaging. This trial suggests the opposite may be true for people with psychotic disorders.
Questions still open
- Why did CBD exacerbate rather than protect against THC effects in this population? Do patients with schizophrenia metabolize cannabinoids differently? Should harm reduction messaging about CBD be revised?
Common questions
Does CBD protect against THC's effects in schizophrenia?
Could this be explained by higher THC levels?
Read the original research
Does cannabidiol reduce the adverse effects of cannabis in schizophrenia? A randomised, double-blind, cross-over trial.
Neuropsychopharmacology : official publication of the American College of Neuropsychopharmacology, 50(12), 1759-1767
Citation
Chesney, Edward; Oliver, Dominic; Sarma, Ananya; Lamper, Ayşe Doğa; Slimani, Ikram; Lloyd, Millie; Dickens, Alex M; Welds, Michael; Kråkström, Matilda; Gasparini-Andre, Irma; Orešič, Matej; Lawn, Will; Babayeva, Natavan; Freeman, Tom P; Englund, Amir; Strang, John; McGuire, Philip. (2025). Does cannabidiol reduce the adverse effects of cannabis in schizophrenia? A randomised, double-blind, cross-over trial.. Neuropsychopharmacology : official publication of the American College of Neuropsychopharmacology, 50(12), 1759-1767. https://doi.org/10.1038/s41386-025-02175-3
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