A substantial proportion of patients with schizophrenia has co-morbid psychoactive substance abuse, which can influence their cognitive functions. The aim of the study is an assessment of cognitive functioning in abstinent schizophrenia patients with various previous pattern of psychoactive substance abuse.

The study was performed on a group of 80 schizophrenia patients (74 male, 6 female), aged 18–40 (mean 25) years, of whom in 40, a co-morbid psychoactive substance abuse was diagnosed. The latter group was subdivided, based on their predominant type of substance (opioid, amphetamine, or cannabis). All patients were examined during clinical improvement, and patients with comorbid substance abuse, after a six-week period of detoxification in a therapeutic community. A battery of neuropsychiatric tests was used, which included subtests of Trail Making Test, Stroop test and Verbal Fluency Test.

No significant differences in clinical factors and cognitive functioning between two examined groups were found. However, when the patients were divided according to their pattern of substance addiction, it turned out that the group of patients with cannabis, despite the shortest duration of disease and that of addiction, and highest percentage of using atypical antipsychotics, performed worse on all cognitive tests, significantly so on Stroop's and Fluency tests, compared to the groups with predominant opioid or amphetamine abuse.

Abstinent schizophrenic patients who previously abused cannabis have worse cognitive functioning compared to other schizophrenic patients with comorbid substance abuse. A possible role of previous cannabis use or cannabis withdrawal in this phenomenon is discussed.