A CBD/THC drug (100mg CBD + 3.3mg THC) administered for about 16 days caused small but measurable declines in verbal fluency and more cognitive adverse events compared to placebo in Parkinson's patients.
Read this if you have Parkinson's disease and are considering cannabis-based treatments, or if you are a clinician advising PD patients about cannabis.
2x cognitive AEsCBD/THC group reported at least twice as many cognitive adverse events as placebo
What the researchers found
In a randomized, double-blind trial of 58 Parkinson's disease patients, the CBD/THC group (n=29) performed worse than placebo (n=29) on the Animal Verbal Fluency test after adjusting for age and education. Cognitive adverse events were reported at least twice as often in the CBD/THC group. The drug contained high-dose CBD (100mg) with low-dose THC (3.3mg) and was escalated to twice daily dosing.
Why it matters
Parkinson's patients are already at elevated risk for cognitive decline. This study provides important safety data showing that even a CBD-dominant cannabis formulation can measurably impact cognition in this vulnerable population.
The numbers in context
- 58 participants: 29 CBD/THC, 29 placebo
- Drug: 100mg CBD + 3.3mg THC per dose
- Treatment duration: 16.3 days (SD 4.2)
- Escalated to twice daily
- Significantly worse Animal Verbal Fluency in CBD/THC group
- Cognitive adverse events at least 2x more frequent in CBD/THC group
How the study worked
Phase IIb randomized, double-blind, parallel-group, placebo-controlled study. Neuropsychological tests administered at baseline and 1-1.5 hours after final dose. Longitudinal regression models adjusted for age and education. Alpha = 0.05.
Who was studied
58 Parkinson's disease patients in a randomized controlled trial
What this study cannot tell us
Cognitive testing occurred 1-1.5 hours after final dose, capturing acute drug effects rather than lasting cognitive changes. Short treatment duration. Small sample size. Single verbal fluency measure showed significance; other cognitive tests may not have been affected.
How to read the evidence
Phase IIb randomized double-blind placebo-controlled trial. Strong design with adequate sample for safety signal detection.
When this study was published
Published in 2023. Among the first rigorous cognitive safety studies of cannabis in Parkinson's disease.
The bigger picture
The cannabis-for-Parkinson's movement is driven by patient demand, but cognitive safety data has been largely absent. This trial provides concrete evidence that clinicians need to weigh potential symptom benefits against measurable cognitive costs, particularly in a disease where cognitive decline is already a major concern.
Questions still open
- Would the cognitive effects reverse after discontinuing the CBD/THC drug?
- Is the verbal fluency decline clinically meaningful or just statistically significant?
- Would CBD without any THC show the same cognitive effects?
Read the original research
Cognitive Safety Data from a Randomized, Double-Blind, Parallel-Group, Placebo-Controlled Phase IIb Study of the Effects of a Cannabidiol and Δ9-Tetrahydrocannabinol Drug on Parkinson's Disease-Related Motor Symptoms.
Movement disorders : official journal of the Movement Disorder Society, 38(7), 1341-1346
Citation
Domen, Christopher H; Sillau, Stefan; Liu, Ying; Adkins, Michelle; Rajkovic, Sarah; Bainbridge, Jacquelyn; Sempio, Cristina; Klawitter, Jost; Leehey, Maureen A. (2023). Cognitive Safety Data from a Randomized, Double-Blind, Parallel-Group, Placebo-Controlled Phase IIb Study of the Effects of a Cannabidiol and Δ9-Tetrahydrocannabinol Drug on Parkinson's Disease-Related Motor Symptoms.. Movement disorders : official journal of the Movement Disorder Society, 38(7), 1341-1346. https://doi.org/10.1002/mds.29447