How does cognition change over time in young people who use cannabis recreationally? A narrative review of the longitudinal literature to date

INTRODUCTION: The regular consumption of cannabis has been associated with changes in cognition. However it is unclear how cognitive performance changes over time with continued cannabis use and whether cognitive alterations predate the onset of cannabis use. We conducted a narrative review of longitudinal observational naturalistic studies of cognition in people who use cannabis. METHODS: We identified studies through PubMed and PsycINFO via the search terms "cannabis", "marijuana", "cognitive", and "cognition" and summarised findings by cognitive domain. RESULTS: The evidence included 21 studies of ~ 27,734 participants aged 5-to-42 years at baseline, comprising 11,783 people who consume cannabis - of which ~ 2,030 who were cannabis naïve at baseline and commenced consuming cannabis at later timepoints - and 15,951 controls. Consistent evidence (≥ 65% significant) showed: decreased IQ in the cannabis group vs. controls over time; associations between chronicity of use (e.g., persistent use/dependence, greater amount/frequency), lower IQ at distinct timepoints, and IQ decline over time; negative correlations between greater frequency of use at distinct timepoints and lower executive function; and negative correlations between increased frequency of use over time and decreases in executive function. There was mixed evidence (40%-to-60% significant) for differences between cannabis vs. control groups at individual timepoints and over time, for: verbal learning and memory, working/episodic memory, and perceptual reasoning. There was minimal evidence (≤ 35% significant) for cannabis use-related changes over time in verbal fluency/comprehension and processing speed, and insufficient evidence for attention. There was insufficient and mixed evidence from 6 studies comparing inconsistent cognitive domains prior to the onset of cannabis use, in cannabis naïve youth. CONCLUSIONS: Continued cannabis consumption over time and greater/increased frequency of cannabis use may affect performance in select domains of cognition. Evidence on how cognition changes in vulnerable people who use cannabis across the lifespan, and in relation to ceasing consumption is required.