Background: Effect of cannabis use on memory function is a contentious issue, with effects being different in healthy individuals and patients with psychosis.
Method: Employing a meta-analytic approach we investigated the effects of cannabis use on memory function in patients with psychosis and healthy individuals, and the effect of diagnosis, memory dimension and moderating factors. A total of 88 studies were identified through a systematic literature search, investigating healthy (n = 7697) and psychotic (n = 3261) individuals. Standardized mean differences between the cannabis user and non-user groups on memory tasks were estimated using random-effects models and the effect-size statistic Cohen's d. Effects of potential moderating factors were tested using mixed-effects models and subgroup analyses.
Results: We found that cannabis use was associated with significantly (p ⩽ 0.05) impaired global (d = 0.27) and prospective memory (d = 0.61), verbal immediate (d = 0.40) and delayed (d = 0.36) recall as well as visual recognition (d = 0.41) in healthy individuals, but a better global memory (d = -0.11), visual immediate recall (d = -0.73) and recognition (d = -0.42) in patients. Lower depression scores and younger age appeared to attenuate the effects of cannabis on memory. Cannabis-using patients had lower levels of depression and were younger compared with non-using patients, whilst healthy cannabis-users had higher depression scores than age-matched non-users. Longer duration of abstinence from cannabis reduced the effects on memory in healthy and patient users.
Conclusions: These results suggest that cannabis use is associated with a significant domain-specific impairment in memory in healthy individuals but not in cannabis-using patients, suggesting that they may represent a less developmentally impaired subgroup of psychotic patients.
Keywords: Cannabis; memory; meta-analyses; psychosis; Δ-9-tetrahydrocannabinol.