30th August 2021
By Emily Ledger
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Medical cannabis use has been consistently on the rise over recent years as a growing number of countries introduce legislation to legalise production and supply of cannabis-based products. These products can now be prescribed in many countries – including the UK – for a large number of conditions, including chronic pain, multiple sclerosis, and epilepsy.

While there have been extensive studies to assess the relationship between recreational cannabis use and cognitive function, there has been little research into how the use of medical cannabis products can affect cognition. Numerous studies have found that earlier onset of recreational cannabis use is associated with poorer cognitive performance, likely due to exposure at a period of neurodevelopmental vulnerability.

Delta-9-tetrahydrocannabinol (THC) – the primary intoxicating cannabinoid produced by the cannabis plant – has been linked to a disruption in neural development, particularly in adolescence. 

On the other hand, the majority of medical cannabis patients initiate treatment during adulthood. Therefore, recreational cannabis users and medical cannabis patients may experience different cognitive effects. An ongoing, longitudinal study aimed to examine the potential long-term effects of medical cannabis use on cognition.  

Design and Methods of the Study 

The study was designed with an aim to track the long-term impact of medical cannabis treatment with the use of cognitive and clinical assessments. These assessments were carried out a baseline, prior to the initiation of medical cannabis treatment, and at multiple intervals throughout the study period (months 3, 6, and 12). 

To date, 54 medical patients have been successfully enrolled and completed at least one follow-up assessment. While the study is ongoing, the researchers have published preliminary findings based on these participants. Of the 54 medical cannabis patients, 51 completed 3-month follow-up, 44 completed a 6-month follow-up, and 32 completed a 12-month follow-up. 

Based on the fact that medical cannabis users are often older and may choose non-intoxicating products or products with a more varied cannabinoid profile, the researchers hypothesised that this cohort would not demonstrate decrements in cognitive function and would instead, exhibit improvements after 12 months of treatment. 

Preliminary Findings of the Study 

The researchers compared data from each follow-up to those from baseline assessments and bivariate correlation analyses were used to assess the relationship between changes in cognitive performance and clinical ratings, as well as the association between these variables and cannabinoid exposure. 

Chronic pain was the most commonly reported reason for medical cannabis use (n=36), followed by anxiety or PTSD (n=33), sleep (n=22), mood (n=14), and attention (n=4). Thirty-six patients reported using medical cannabis for more than one condition. Participants reported using cannabis products, on average, 9-11 times per week. Cannabinoid exposure calculations revealed that overall, THC exposure was significantly lower than CBD exposure at each visit. However, this difference was only statistically significant for medical cannabis use after 6 months of treatment.  

Using the Stroop Colour and Word Test (SCWT) – a neuropsychological test used to assess cognitive measures such as selective attention, automaticity, inhibitory processes, and executive control – medical cannabis patients exhibited significantly faster times at all follow-up visits, compared with baseline, suggesting improved inhibitory processing. 

While participants demonstrated high levels of accuracy across visits, a statistically significant reduction in accuracy was noted at 3 months. However, the researchers conclude that, given the high levels of accuracy attained and the very small change from baseline to three months (<1%), this didn’t appear to be clinically significant. Furthermore, this pattern was not seen to continue to later visits, suggesting that improvements in Stroop completion times “did not come at the expense of lower task accuracy.” 

Participants also generally exhibited improved performance in the Wisconsin Card Sorting Test, following the initiation of medical cannabis treatment. Perseverative errors generally decreased across visits; however, this was only statistically significant following 12 months of treatment.  

Patients were also asked to complete Letter-Number Sequencing (LNS) tasks – a subtest of the Wechsler Adult Intelligence Scale (WAIS) that measures short-term memory skills in being able to process and re-sequence information. Statistically significant improvements were seen at all three follow-up visits relative to baseline. 

Conclusions 

In contrast to studies of recreational cannabis users, which reported decrements in cognitive performance – particularly among consumers with adolescent onset -, the preliminary results from this ongoing study suggest that 3-12 months of medical cannabis treatment does not appear to be associated with poorer cognitive performance. Furthermore, these findings also expand upon pilot findings which indicated improvement on some measures of executive function following 3 months of medical cannabis. 

The researchers also note that improvements in cognitive performance in the current study appear to occur in the context of significant improvements on measures of mood, anxiety, and sleep. These findings are supported by retrospective reports of clinical improvements secondary to medical cannabis use among various patient populations.  

While greater improvement of clinical state over time was significantly associated with increased CBD exposure (mg/week), improved cognitive performance over time did not appear to correlate with medical cannabis use. The researchers conclude that these preliminary finings warrant the initiation of further investigations to examine the impact of individual cannabinoids and age of onset of use.  

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