The effects of cannabis on memory function in users with and without a psychotic disorder: findings from a combined meta-analysis.
BACKGROUND
Effect of cannabis use on memory function is a contentious issue, with effects being different in healthy individuals and patients with psychosis.


METHODS
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.
Department of Psychosis Studies,Institute of Psychiatry, Psychology & Neuroscience,King's College London,De Crespigny Park,London SE5 8AF,UK.
T
T
Schoeler
Department of Psychosis Studies,Institute of Psychiatry, Psychology & Neuroscience,King's College London,De Crespigny Park,London SE5 8AF,UK.
J
J
Kambeitz
Department of Psychosis Studies,Institute of Psychiatry, Psychology & Neuroscience,King's College London,De Crespigny Park,London SE5 8AF,UK.
I
I
Behlke
Department of Psychosis Studies,Institute of Psychiatry, Psychology & Neuroscience,King's College London,De Crespigny Park,London SE5 8AF,UK.
R
R
Murray
Department of Psychosis Studies,Institute of Psychiatry, Psychology & Neuroscience,King's College London,De Crespigny Park,London SE5 8AF,UK.
S
S
Bhattacharyya
D000328:Adult; D063386:Cannabinoid Receptor Agonists / Q000009:adverse effects / Q000494:pharmacology*; D013759:Dronabinol / Q000009:adverse effects / Q000494:pharmacology*; D006801:Humans; D008568:Memory / Q000187:drug effects*; D008569:Memory Disorders / Q000139:chemically induced*; D011618:Psychotic Disorders / Q000503:physiopathology*
D063386:Cannabinoid Receptor Agonists; D013759:Dronabinol
Cannabis; memory; meta-analyses; psychosis; Δ-9-tetrahydrocannabinol