The effect of medical cannabis on cognitive functions: a systematic review
What this study found
Overall, the evidence was inconclusive and did not support a single clear effect of cannabis-based medicines on cognition. Several higher-quality studies suggested that any cognitive harm was small when THC doses were low to moderate, while CBD alone did not appear to severely affect cognition at the doses used. Some studies reported domain-specific impairment, including PASAT worsening with smoked or vaporized cannabis and a dual-task Stroop effect in multiple sclerosis, but other studies showed no significant impairment or even improved scores such as SDMT and CVLT in one long-term Sativex…
- Study & population
- This was a systematic review of 23 controlled studies including 917 participants receiving active cannabis-based interventions.
- Intervention
- Cannabis-based medicines were studied in multiple formulations, including THC/CBD sprays, smoked or vaporized cannabis, oral capsules, and full-spectrum oils.
- Key limitation
- The review was limited by substantial heterogeneity in cannabis formulation, THC/CBD ratio, route of administration, dose, and treatment duration.
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Open PDFOriginal abstract
Background Cannabis-based medicines are widely used in the treatment of a number of medical conditions. Unfortunately, cognitive disturbances are often reported as adverse events, although conversely, cognitive improvements have been reported. Hence, the objective of the present study was to identify, critically appraise and synthesise research findings on the potential impact of cannabis-based medicines on cognitive functioning. Methods Four databases (EMBASE, PsycINFO, PubMed and Scopus) were systematically searched. Studies were included if they provided findings on the impact of cannabis-based medicines in controlled settings on cognitive functioning measured by recognised cognitive tests in human adults. Study participants were required to be their own case-control, and neither studies on abuse, abstinences, patients with severe neurodegenerative diseases nor cancer-related pain conditions were included. Screening, risk of bias assessment and data extraction were conducted independently by two researchers. Findings were tabulated and synthesised by outcome. Findings Twenty-three studies were included, comprising a total of N = 917. Eight studies used Sativex as the cannabis-based medicine two used Epidiolex, two other studies used sprays, three studies used gelatine capsules, five smoked cannabis, two other and finally one studied cannabis withdrawal. Fifteen studies reported non-significant findings; six reported cognitive impairments; one study found cognitive improvement and a single study found improvement following withdrawal. Thirteen studies had cognitive or neuropsychological functioning as the primary outcome. Conclusions Due to a large heterogeneity and methodological limitations across studies, it is not possible to make any definite conclusions about the impact of cannabis-based medicines on cognitive functioning. However, the majority of high-quality evidence points in the direction that the negative impact of cannabis-based medicines on cognitive functioning is minor, provided that the doses of THC are low to moderate. On the other hand, long-term use of cannabis based medicines may still adversely affect cognitive functioning. In the studies that found impaired cognitive functioning to be significant, all of the test scores were either within the normal range or below what would be characterised as a neuropsychologically cognitive impairment.