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What Effect Does Cannabis Have on the Brain?

Published: 22/02/2022

Decades of research have sought to establish the effects of cannabis on cognitive abilities of humans. Most of this research has focused upon recreational cannabis, however little has been established on the effects of medical cannabis on cognitive abilities. Recreational cannabis consumption during adolescence has shown an association with reduced cognitive performance. Whether this is causative or merely an association, however, is debated.

There are thought to be distinct differences between medical cannabis and recreational cannabis consumers which may grossly impact the effects on cognition. Medical cannabis patients are typically older compared to recreational cannabis consumers, who typically start consuming cannabis during adolescence. Recreational cannabis typically contains higher quantities of tetrahydrocannabinol (THC), whilst medical cannabis typically has higher concentrations of cannabidiol (CBD) which may negate some of the associated adverse effects of THC.

Whilst an association exists between recreational cannabis consumption in adolescence and reduced cognitive performance, it is also known that the presence of chronic conditions such as pain, poor sleep and anxiety are also known to affect cognitive performance. A recent study aimed to assess whether there is any associated cognitive decline with medical cannabis therapy.


The study recruited 54 patients, who were 21 years of older with a valid medical cannabis license. All patients had to be naïve to cannabis or had limited exposure over the previous year, confirmed on urinary testing. Each patient also was required to have an IQ greater than or equal to 75.

Each patient was subject to several scores that assessed neurocognitive abilities at baseline, 3 months, 6 months, and 12 months. The neuropsychological battery of tests included:

  • Assessment of executive function
    • Stroop Colour Word Test
    • Trail Making Test
    • Wisconsin Card Sorting Test
    • Letter-Number Sequencing (LNS) subtest of the Wechsler Adult Intelligence Scale-Revised
    • Controlled Oral Word Association Test
  • Assessment of verbal learning and memory
    • Rey Auditory Verbal Learning Test

In addition, patients were also asked to complete the following tests to asses clinical effects on anxiety, depression, and sleep:

  • Profile of Mood State
  • Beck Anxiety Inventory
  • Beck Depression Inventory
  • State Trait Anxiety Index
  • Pittsburgh Sleep Quality Index

Cannabinoid products were independently tested for cannabinoid content or certificates of analysis were used to assess CBD and THC doses consumed per week.


Of the 54 total patients, 51, 44, and 32 completed 3-, 6-, and 12-month analysis. 12 (22%) stopped participation in the study. 3 patients stopped using medical cannabis, whilst 9 were lost to follow up. 36 patients used medical cannabis for more than one condition including pain (n = 36), anxiety or PTSD (n = 31), sleep (n = 22), mood (n = 14), attention (n = 4), and other medical conditions (n = 4). The average age of participants was 49.

Assessment of cognitive changes demonstrated a range of effects. Some findings showed a significant improvement in outcomes, whilst others showed no change or a reduction in cognitive performance. However, on balance the cognitive changes appeared to stay stable or a slight trend towards improvement.

Across all clinical outcomes there were significant improvements in total mood disturbance, symptoms of depression, symptoms of anxiety, and sleep quality at all time periods.

There were no correlations identified between doses of THC or CBD with cognitive outcomes. However, there were significant associations with improved clinical outcomes and higher doses of CBD per week. There was a correlation seen between some assessments of cognitive function and improved clinical variables.


In summary, this study demonstrated no decline in cognitive effects over short-term follow up. In fact, some improvements were visualised in specific domains. Other domains had statistically significant declines in cognitive performance, however these were deemed to be not clinically significant. Improvements in clinical variables following administration medical cannabis were associated with improvement in cognitive performance.

There are however limitations of this study. Firstly, the average age of the population was 49 and therefore these findings may not be extrapolated to adolescent/paediatric population. This was also a predominantly white, non-diverse population from an affluent US state. Moreover, there was no comparison group to assess how cognition changes over 12 months in similar patients not receiving medical cannabis.

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