The impact of cannabis use on cognitive functioning in patients with schizophrenia: a meta-analysis of existing findings and new data in a first-episode sample

Abstract

Cannabis use is highly prevalent among people with schizophrenia, and coupled with impaired cognition, is thought to heighten the risk of illness onset. However, while heavy cannabis use has been associated with cognitive deficits in long-term users, studies among patients with schizophrenia have been contradictory. This article consists of 2 studies. In Study I, a meta-analysis of 10 studies comprising 572 patients with established schizophrenia (with and without comorbid cannabis use) was conducted. Patients with a history of cannabis use were found to have superior neuropsychological functioning. This finding was largely driven by studies that included patients with a lifetime history of cannabis use rather than current or recent use. In Study II, we examined the neuropsychological performance of 85 patients with first-episode psychosis (FEP) and 43 healthy nonusing controls. Relative to controls, FEP patients with a history of cannabis use (FEP + CANN; n = 59) displayed only selective neuropsychological impairments while those without a history (FEP - CANN; n = 26) displayed generalized deficits. When directly compared, FEP + CANN patients performed better on tests of visual memory, working memory, and executive functioning. Patients with early onset cannabis use had less neuropsychological impairment than patients with later onset use. Together, these findings suggest that patients with schizophrenia or FEP with a history of cannabis use have superior neuropsychological functioning compared with nonusing patients. This association between better cognitive performance and cannabis use in schizophrenia may be driven by a subgroup of "neurocognitively less impaired" patients, who only developed psychosis after a relatively early initiation into cannabis use.

Fig. 1.

Effect Size Analysis of All Studies Conducted To Date. A summary score global cognition index was calculated by averaging effect sizes from each domain for each study.

Fig. 2.

Mean Z Score Differences for Cannabis Using and Nonusing FEP Groups. Note: IQ, intelligence; PS, processing speed; verMEM, verbal memory; visMEM, visual memory; wMEM, working memory; EF, executive functioning. Note that each point reflects a specific task within the 6 domains of interest (refer to the Methods section and table 5 for the specific tasks).

Fig. 3.

Mean Z Score Differences for Early and Late Cannabis Using FEP Groups. Note: IQ, intelligence; PS, processing speed; verMEM, verbal memory; visMEM, visual memory; wMEM, working memory; EF, executive functioning. Note that each point reflects a specific task within the 6 domains of interest (refer to the Methods section and table 5 for the specific tasks).