MS Society supported study investigates relationship between cannabis use and cognitive dysfunction in MS


Cognitive dysfunction affects 40-60% of individuals with MS. Common signs of cognitive dysfunction include decreased memory, concentration, and attention, as well as difficulty putting thoughts into words. It has been shown through functional magnetic resonance imaging (fMRI) that in individuals with MS, additional regions of the brain are activated while they are performing cognitive tasks. This is presumed to occur as a type of compensation for the structural abnormalities that are present. fMRI works by looking at blood flow in the brain in real time to detect areas of activity. Where there is increased activity, blood flow to that region increases. This technique is unique in that it can observe how the brain functions, compared to standard MRI which can only detect structural abnormalities. 

Cannabis is sometimes used by people with MS as a type of treatment to offer relief from symptoms such as pain, spasticity, and tremor. As a treatment, cannabis can be taken orally or smoked. Recently new research has surfaced evaluating the benefits of smoking cannabis in relieving symptoms. However, some studies have reported cognitive challenges in people with MS who smoke cannabis for medical purposes. An MS Society funded study recently conducted in Toronto used fMRI to investigate how smoked cannabis affects cognition in individuals living with MS.

The Study:

MS Society funded researcher and psychiatrist Dr. Anthony Feinstein and colleagues conducted a study with 39 subjects diagnosed with MS. Twenty of the subjects regularly smoked cannabis (but had not done so in the 12 hours preceding the test), while the other 19 were nonusers. All subjects underwent fMRI while performing three different versions of a working memory test known as N-Back. The researchers compared brain activity between the two groups while the subjects completed the tests. The researchers also collected data on resting-state fMRI, which measures brain activity while the subjects were not completing any tasks. Structural MRI data was gathered to evaluate any differences in brain structure between the two groups. Additional tests were administered to obtain data on measures of verbal and visual memory, information processing speed, and attention.


The researchers found that individuals with MS who smoked cannabis  performed less well than individuals with MS who did not smoke cannabis on the third version of the N-Back test - known as the 2-Back - which is more difficult than the first two versions. However, while the cannabis group obtained fewer correct answers, their reaction times were the same as the nonusers. In addition, fMRI results showed abnormal patterns of brain activity in the cannabis group. There were no differences found between the two groups in the data collected from the resting-state fMRIs and structural MRIs. The data collected from the other tests showed that the cannabis group performed more poorly on the 2-second Paced Auditory Serial Addition Test (PASAT), which measures information processing speed, and on a visual test.


With more people exploring the use of alternative and complementary approaches such as cannabis for treating MS symptoms, it becomes increasingly important to determine the risks of the treatment versus the benefits. This study adds to the growing body of scientific evidence demonstrating the effects of cannabis in people living with MS. It demonstrates that smoking cannabis may add to the cognitive challenges experienced by people with MS, and researchers now have imaging data which could explain this observation. The disorderly pattern of brain activation in the cannabis users may suggest that the brain is compensating, or working much harder, to overcome the tasks required. Results from the study may help to better inform decisions around the prescription and use of medical cannabis, but also pave the way for the development of imaging approaches that can more accurately measure cognitive deficit in people with MS.


Pavisian B et al. Effects of cannabis on cognition in patients with MS. Neurology 2014 April 30 [Epub ahead of print]