Cognitive function in people who use cannabis and in those who endorse a cannabis use disorder

<p dir="ltr">Cannabis use and cannabis use disorder (CUD) are highly prevalent and can be characterised by persistent, compulsive use despite stress and impairment. The negative outcomes of cannabis use and CUD have partly been ascribed to altered cognition as posited by models of addiction. Yet to date, there is a paucity of research that has comprehensively examined cognition in young adults who consume cannabis and endorse a CUD. The first two studies of this thesis aim to address this knowledge gap by: (i) comparing performance in a range of cognitive domains between regular cannabis users who endorse a CUD and non-using controls, and (ii) examining associations between level of problematic cannabis use (Cannabis Use Disorder Identification Test – Revised; CUDIT-R), cannabis dosage (cannabis grams/past month), and cognitive outcomes, accounting for confounds (alcohol/nicotine use, anxiety symptoms, hours since last cannabis use). The sample comprised 115 participants (83 CUD vs 32 controls) aged 18.5 to 32.5 years which I recruited from the general community with a research team.</p><p dir="ltr">Study 1 examined a broad range of cognitive domains: executive function, working memory, episodic memory, verbal reasoning, attention, IQ. Results indicated that IQ was lower in the CUD group compared to controls (p < .001, d = 0.862); this effect was driven by lower verbal IQ which survived additional adjustment for years of education. Other cognitive domains did not differ between groups, nor did CUDIT-R or cannabis dosage predict any domain of cognition in the CUD group. Overall, the findings from Study 1 suggests that worse cognition in young adults with a CUD may be confined to verbal IQ. Study 2 examined a range of social cognitive domains: emotion recognition/differentiation, face memory. Social cognition was not significantly affected by group status, nor by CUDIT-R or cannabis dosage. Thus, altered social cognition might not be a core feature in young adults who endorse a CUD. Overall, findings from Study 1 and 2, together with the evidence to date, may indicate that cognitive alterations in CUD are not as robust as reported in other substance use disorders, and as posited by theories of addiction. Alternatively, some moderators might have protected from cognitive alterations including young age, above average IQ, and lack of treatment seeking.</p><p dir="ltr">A substantial number of people who use cannabis continue consumption over time. Yet, there is a knowledge gap regarding how continued cannabis use affects cognition over time. The longitudinal evidence on cannabis use and cognition needs to be summarised as past reviews are outdated and focus on select ages. Study 3 is a narrative review of the longitudinal evidence to date on cannabis use and cognition across the lifespan. 21 studies were identified that examined ~21,371 participants (8,107 cannabis users, 13,264 controls) aged 5-to-42 years at baseline and followed up 2-to-9 times. Eight studies compared ~2,304 cannabis naïve individuals before they commenced using cannabis at follow-up and ~3,995 controls. Findings were summarised per cognitive domain: working/episodic memory, executive function, verbal fluency/comprehension, IQ, processing speed, perceptual reasoning, verbal learning and memory, attention.</p><p dir="ltr">There was mixed evidence (40%-to-60% significant) that reduced IQ predated cannabis use, and insufficient evidence that other cognitive domains (e.g., working memory, executive function) were altered prior to cannabis use onset. There was consistent evidence ( 65% significant) for: (i) decreased IQ in cannabis users vs controls over time, (ii) associations between patterns of use (e.g., persistent use/dependence, greater dosage/frequency) and lower IQ at distinct timepoints/IQ decline over time, and (iii) associations between greater frequency at individual timepoints/increased frequency over time and decreased executive function. The evidence was mixed regarding cognitive decline over time in cannabis users compared to controls for: verbal learning and memory and perceptual reasoning and minimal ( 35% significant) or insufficient (< 3 studies) evidence for cannabis-related changes in other cognitive domains: working/episodic memory, verbal fluency/comprehension, processing speed, and attention. The longitudinal evidence to date may be biased by over representation of youth samples and limited by inconsistent metrics of cannabis use and cognition.</p><p dir="ltr">Overall, the findings from this thesis indicate that verbal IQ may be a feature of CUD and IQ decline may be associated with continued cannabis use and with continued, greater chronicity of use. Future work is required to confirm whether the findings from this thesis generalise to CUD samples across the lifespan, including the most vulnerable individuals with a CUD who are seeking/receiving treatment and that endorse comorbid psychopathologies.</p>