Cannabis use is associated with cognitive impairment in patients with schizophrenia, and is linked to poor social and employment outcomes in this disorder. 

A new study published online in the December 2012 issue of Psychiatry Research, led by CAMH Scientist Dr. Tony George and PhD student Rachel Rabin, suggests that in people with schizophrenia, increasing exposure to cannabis is associated with impairment of various types of memory, attention and decision-making tasks linked to functional outcomes. 

Furthermore, when cannabis-dependent patients with schizophrenia abstained from cannabis misuse for at least 6 months, these cognitive impairments disappeared and returned to baseline.

“Generally, cannabis misuse by people with schizophrenia is associated with poor outcomes such as symptom exacerbation, medication noncompliance and increased hospital admissions, but direct evidence for cognitive impairment has been mixed," said Dr. George, who is Chief of CAMH’s Schizophrenia Division, Medical Director of the Complex Mental Illness Program and Senior Scientist in the Campbell Family Mental Health Research Institute. "This study is important because it suggests that quitting cannabis use in people with schizophrenia is associated with cognitive improvements which directly affects psychosocial functioning in these patients." 

Results from this study of 47 cannabis-dependent males with schizophrenia suggest that “there is hope that poor cognitive performance can be improved by quitting marijuana in those who were once heavy chronic users” explained Rabin, first author of the study. 

Rabin and George plan to follow-up on this preliminary finding, and assess performance in individuals with schizophrenia compared to non-psychiatric participants abstaining marijuana for up to one month. Further research with more patients would strengthen these conclusions.