Despite the controversy surrounding the possible causal link between cannabis use and the onset of schizophrenia (SZ), data seeking to elucidate the effect of cannabis use disorders (CUDs) on the clinical presentation of SZ have produced mixed results. Although several studies have suggested that CUD in patients with SZ may be associated with variation in cognitive function, clinical presentation and course of illness, the effects have been inconsistent.
We retrospectively ascertained a large cohort (N=455) of SZ patients with either no history of a CUD (CUD-; N=280) or a history of CUD (CUD+; N=175). The groups were initially compared on key demographic variables including sex, race, age, age at onset of SZ, parental socioeconomic status, premorbid IQ, education level and global assessment of functioning. Covarying for any observed differences in demographic variables, we then compared groups on lifetime measures of psychotic symptoms as well as a brief battery of neurocognitive tests.
Compared to the CUD- group the CUD+ group demonstrated significantly better performance on measures of processing speed (Trail Making Tests A and B), verbal fluency (animal naming) and verbal learning and memory (California Verbal Learning Test). Moreover, the CUD+ group had better GAF scores than the CUD group.
Collectively, these findings suggest that SZ patients with comorbid CUD may represent a higher functioning subgroup of SZ. Future prospective studies are needed to elucidate the nature of this relationship.