The effects of cannabis on memory function in users with and without a psychotic disorder: findings from a combined meta-analysis

Original Articles

T. Schoeler^a1, J. Kambeitz^a1a2, I. Behlke^a1a3, R. Murray^a1 and S. Bhattacharyya^a1 c1

^a1 Department of Psychosis Studies, Institute of Psychiatry, Psychology & Neuroscience, King’s College London, De Crespigny Park, London SE5 8AF, UK

^a2 Department of Psychiatry, Ludwig-Maximilian-University Munich, Nuβbaumstr. 7, 80336 Munich, Germany

^a3 Institute of Psychology, Univeristy of Osnabrueck, Seminarstr. 20, 49074 Osnabrueck, Germany

Abstract

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.

(Received September 03 2014)

(Revised July 29 2015)

(Accepted August 04 2015)

Key words

• Cannabis;
• memory;
• meta-analyses;
• psychosis;
• Δ-9-tetrahydrocannabinol

Correspondence

Address for correspondence: S. Bhattacharyya, M.B.B.S., M.D., Ph.D., Department of Psychosis Studies, Institute of Psychiatry, King’s College London, De Crespigny Park, London SE5 8AF, UK. (Email: sagnik.2.bhattacharyya@kcl.ac.uk)