Cannabis use and cognitive function: 8-year trajectory in a young adult cohort

Aim To evaluate the relationship between change in cannabis use and changed cognitive performance over 8 years. Design We used survey methodology with a cohort design. Setting and participants An Australian community sample aged 20–24 years at baseline. Measures We assessed cognitive performance with the California Verbal Learning Test (CVLT) (immediate and delayed), Spot‐the‐Word test (STW), Symbol Digit Modality test (SDMT) and Digit Backwards (DB). Groups of cannabis users were defined from self‐reports across three waves as: ‘never’ (n= 420) ‘remain light’ (n= 71), ‘former light’ (n= 231), ‘remain heavy’ (n= 60), ‘former heavy’ (n= 60) and ‘always former’ (since start of study) (n= 657). Planned contrasts within mixed model repeated‐measures analysis of variance was used for longitudinal analysis with an adjusted alpha of 0.01. Findings Data were obtained from 2404 participants with 1978 (82.3%) completing wave 3. At baseline there were significant differences between cannabis groups on CVLT (immediate and delayed) and SDMT. However, after controlling for education, gender, gender × group and gender × wave, there were no significant between‐group differences and only CVLT immediate recall reached adjusted statistically significant longitudinal change associated with changed cannabis use (group × wave P= 0.007). Specifically, former heavy users improved their performance relative to remaining heavy users (estimated marginal means: former heavy 6.1–7.5: remain heavy 6.4–6.6). Conclusions Cessation of cannabis use appears to be associated with an improvement in capacity for recall of information that has just been learned. No other measures of cognitive performance were related to cannabis after controlling for confounds.     

Cannabis and educational achievement

Aims To examine the relationship between cannabis use in adolescence/young adulthood and levels of educational attainment. Design Data were gathered over the course of a 25‐year longitudinal study of a birth cohort of 1265 New Zealand children. Measurements Measures analysed included (a) frequency of cannabis use in adolescence and young adulthood (15–25 years); (b) levels of educational achievement to age 25 years; and (c) social, family and individual characteristics assessed prior to age 16. Findings Increasing cannabis use was associated with increasing risks of leaving school without qualifications, failure to enter university and failure to obtain a university degree. The association between cannabis use and leaving school without qualifications persisted after control for confounding factors. When due allowance was made for pre‐existing levels of cannabis use there was no evidence to suggest the presence of reverse causal pathways in which lower educational achievement led to increased cannabis use. Conclusions Findings support the view that cannabis use may act to decrease educational achievement in young people. It is likely that this reflects the effects of the social context within which cannabis is used rather than any direct effect of cannabis on cognitive ability or motivation.