Multiple sclerosis patients who smoked cannabis performed worse on cognitive tests and showed more diffuse brain activation patterns than non-using MS patients.
Read this if you have MS and use or are considering cannabis, and want to understand its cognitive effects.
Cannabis users needed more brain activation to attempt working memory tasks and still made more errors
What the researchers found
Compared to matched MS patients who did not use cannabis, cannabis-using MS patients performed significantly worse on two cognitive measures: the more demanding version of the Paced Auditory Serial Addition Test (processing speed) and the 10/36 Spatial Recall Test (visual memory).
On fMRI during a working memory task, cannabis users showed more diffuse cerebral activation across all difficulty levels and made more errors on the most demanding task. During the hardest condition, cannabis users showed increased activation in parietal and anterior cingulate regions involved in working memory, suggesting they needed to recruit more brain resources to attempt the same tasks.
No group differences were found in resting-state brain networks or structural MRI measures (lesion volumes, brain tissue volumes, or white matter integrity), meaning the functional differences were not explained by greater structural brain damage in cannabis users.
Why it matters
MS already compromises cognitive function through brain inflammation and damage. This study suggests cannabis use may compound these cognitive problems by further disrupting the brain's compensatory mechanisms. For MS patients considering cannabis for symptom management, these cognitive trade-offs are important to understand.
The numbers in context
20 cannabis users and 19 non-users with MS. Cannabis users performed worse on PASAT 2-second (p<0.02) and 10/36 Spatial Recall (p<0.03). More errors on 2-Back task (p<0.006). Increased parietal (p<0.007) and anterior cingulate (p<0.001) activation during demanding working memory tasks.
How the study worked
Twenty MS patients who smoked cannabis and 19 non-using MS patients, matched on demographic and neurological variables, underwent fMRI during a working memory task (N-Back), resting-state fMRI, and structural MRI (including diffusion tensor imaging). A comprehensive neuropsychological battery assessed verbal memory, visual memory, processing speed, and attention.
What this study cannot tell us
This was a cross-sectional study, so it cannot determine whether cannabis caused the cognitive differences or whether individuals with more cognitive difficulties were more likely to use cannabis. The cannabis users were assessed in an unintoxicated state, but chronic effects may differ from acute effects. The sample size was small (20 users, 19 non-users).
How to read the evidence
This is a well-designed cross-sectional study with neuroimaging and neuropsychological data, but the sample size was small and the design cannot establish causation.
When this study was published
Published in 2014. Research on cannabis and cognition in MS continues as cannabis use becomes more common in this population.
The bigger picture
MS patients often use cannabis for pain, spasticity, and sleep. However, this study indicates that cannabis use may worsen the cognitive difficulties that are already common in MS. The finding that cannabis disrupts the brain's compensatory activation patterns, without additional structural damage, suggests a functional mechanism that could potentially be reversible with cessation.
Questions still open
- Are the cognitive effects reversible with cannabis cessation? Do different cannabis products (e.g., CBD-dominant) have different cognitive impacts in MS? How should clinicians weigh symptom relief against cognitive risks when counseling MS patients about cannabis?
Common questions
What does "more diffuse activation" mean?
Were the cannabis users intoxicated during testing?
Read the original research
Effects of cannabis on cognition in patients with MS: a psychometric and MRI study.
Neurology, 82(21), 1879-87
Citation
Pavisian, Bennis; MacIntosh, Bradley J; Szilagyi, Greg; Staines, Richard W; O'Connor, Paul; Feinstein, Anthony. (2014). Effects of cannabis on cognition in patients with MS: a psychometric and MRI study.. Neurology, 82(21), 1879-87. https://doi.org/10.1212/WNL.0000000000000446
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