Psychological Medicine

Original Articles

Cannabis and cognitive performance in psychosis: a cross-sectional study in patients with non-affective psychotic illness and their unaffected siblings

J. H. Meijera1, N. Dekkera1, M. W. Koetera2, P. J. Queea3, N. J. M. van Beverena4, C. J. Meijera1 and Genetic Risk and Outcome of Psychosis (GROUP) Investigators c1

a1 Department of Psychiatry, Academic Medical Center, Amsterdam, The Netherlands

a2 Amsterdam Institute for Addiction Research (AIAR), Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands

a3 Department of Psychiatry and Rob Giel Research Center, University Medical Center Groningen, Groningen, The Netherlands

a4 Department of Psychiatry, Erasmus MC, Rotterdam, The Netherlands


Background The relationship between cannabis use and cognitive functioning in patients with psychosis has yielded contradictory findings. In individuals at genetic high risk for psychosis, information is sparse. The aim of this study was to assess the association between recency and frequency of cannabis use and cognitive functioning in patients with psychosis and their unaffected siblings.

Method We conducted a cross-sectional study in 956 patients with non-affective psychosis, 953 unaffected siblings, and 554 control subjects. Participants completed a cognitive test battery including assessments of verbal learning, set shifting, sustained attention, processing speed, working memory, acquired knowledge, reasoning and problem solving and social cognition. Cannabis use was assessed by urinalysis and by the Composite International Diagnostic Interview. Using random-effect regression models the main effects of cannabis (recency and frequency) and the interaction with status (patient, sibling, control) on cognitive functioning were assessed.

Results Current cannabis use was associated with poorer performance on immediate verbal learning, processing speed and working memory (Cohen's d −0.20 to −0.33, p<0.005). Lifetime cannabis use was associated with better performance on acquired knowledge, facial affect recognition and face identity recognition (Cohen's d+0.17 to +0.33, p<0.005). There was no significant interaction between cannabis and status on cognitive functioning.

Conclusions Lifetime cannabis-using individuals might constitute a subgroup with a higher cognitive potential. The residual effects of cannabis may impair short-term memory and processing speed.

(Received March 15 2011)

(Revised June 19 2011)

(Accepted July 22 2011)

(Online publication September 07 2011)


c1 Address for correspondence: L. de Haan (GROUP Investigator), AMC-Academisch Psychiatrisch Centrum, Postbus 75867, 1070 AW Amsterdam, The Netherlands. (Email:


† GROUP Investigators are listed in the Appendix.