Opposite relationships between cannabis use and neurocognitive functioning in bipolar disorder and schizophrenia

Background. Cannabis use is associated with altered neurocognitive functioning in severe mental disorders, but data are still inconclusive and there are no studies of bipolar disorder. The aim of this study was to investigate the association between cannabis use and neurocognition in bipolar disorder compared with schizophrenia in a naturalistic setting. Method. A total of 133 patients with bipolar disorder and 140 patients with schizophrenia underwent neuropsychological assessments and clinical characterization including measures of substance use. Relationships between cannabis users and neurocognitive function were explored in the two diagnostic groups. Possible interactions between diagnosis and cannabis use were investigated, and findings were controlled for possible confounders. Results. In bipolar disorder subjects, cannabis use was associated with better neurocognitive function, but the opposite was the case for the schizophrenia subjects. There was a statistically significant interaction effect of diagnosis and cannabis use on focused attention (p = 0.019), executive functioning (verbal fluency - set shifting) (p = 0.009), logical memory-learning (p = 0.007) and on logical memory-recall (p = 0.004). These differences in neurocognitive function could not be explained by putative confounders. Conclusions. The findings suggest that cannabis use may be related to improved neurocognition in bipolar disorder and compromised neurocognition in schizophrenia. The results need to be replicated in independent samples, and may suggest different underlying disease mechanisms in the two disorders.