BACKGROUND: Cannabis use is highly prevalent among people with psychotic disorders. While there is some indication that cannabis use exacerbates psychosis symptoms, its neurocognitive effects remain unclear. Co-use of cannabis and tobacco is increasing in the general population, but little is known about its effects in people with psychosis or those at risk of developing a psychotic disorder, including its effects on cognitive performance.

METHODS: We analyzed data from the North American Prodrome Longitudinal Study 2, a multi-site prospective study of individuals at clinical high risk for psychosis (CHR-P) and healthy controls. Cognitive performance was assessed using the MATRICS Consensus Cognitive Battery and the Seidman Auditory Continuous Performance Task. We examined differences in cognitive performance across 6 study groups: 1) CHR-P tobacco use, 2) CHR-P cannabis use, 3) CHR-P co-use, 4) CHR-P who did not use tobacco or cannabis, 5) CHR-P who did not use substances, and 6) healthy controls.

RESULTS: Among 1,012 participants (734 CHR-P, 278 healthy controls), co-use of cannabis and tobacco was associated with lower global cognitive performance and lower working memory, verbal learning, and attentional performance compared to healthy controls. CHR-P individuals who did not use substances also had lower cognitive performance across multiple measures, including global cognition, working memory, verbal learning, visual learning, and attentional performance. The CHR-P non-substance use group also had the lowest social function.

CONCLUSIONS: Cannabis and tobacco co-use is associated with lower neurocognitive performance, which may explain the mixed associations between cannabis use and cognitive performance in previous literature.