Cumulative cannabis use is negatively associated with cognition in men with schizophrenia who are currently dependent on the drug, report researchers.
However, Rachel Rabin (University of Toronto, Ontario, Canada) and colleagues found no association between cumulative cannabis use and cognition in schizophrenia patients who were formerly dependent (in remission for at least 6 months).
The findings, published in Psychiatry Research, suggest that "the state dependent negative effects of cannabis may be reversed with sustained abstinence" in schizophrenia patients, say the researchers.
To explore the effects of cannabis use on cognition in schizophrenia, the team recruited 47 male outpatients with the disorder, of whom 18 were currently dependent on cannabis according to DSM-IV criteria. Of the 29 patients who were not current users, 21 had been formerly dependent on the drug and eight had never been dependent.
All participants completed a comprehensive cognitive test battery to assess memory, sustained attention, concentration, psychomotor speed, response inhibition, processing speed, impulsivity, and executive function.
Cross-sectional comparisons revealed that patients who had used cannabis at any point in their lives had better processing speed than those with no lifetime dependence, but there were no significant group differences in other areas of cognition.
Further analysis revealed a "robust" negative association between cumulative cannabis exposure and cognition across various domains in patients who were currently dependent.
No such associations were observed in patients who were not currently dependent on cannabis.
"The present study suggests modest and selective effects of lifetime cannabis dependence on cognitive performance in patients with schizophrenia," conclude Rabin and team.
They add: "While lifetime cannabis users may represent a better functioning subgroup of patients with schizophrenia, cannabis does disrupt cognitive function in that increasing years of cannabis use are associated with worse cognitive performance.
"Given the high prevalence of cannabis misuse combined with the persistence and significance of cognitive deficits in schizophrenia, large-scale longitudinal investigations determining the true effects of cannabis on cognition are necessary."
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