Dual diagnosis of schizophrenic disorders and drug abuse/dependence are increasing due to more frequent use of cannabis and cocaine. It is important to differentiate between primary schizophrenia with associate drug abuse/dependence and drug-induced psychosis. The objective is to detect neuropsychological differences between drug users and non-users in schizophrenia patients, which could be used as diagnostic tools.

We conduct one case-control study on 12 schizophrenia out-patients (10 male) with vital history of drug abuse/dependence (mainly cannabis and cocaine) and one control group of 18 schizophrenia out-patients (12 male) who never used illegal drugs (global age mean: 32.8 years; SD:7.2). It was applied one neuropsychological battery sensitive to the neuropsychological deficit frequent in schizophrenia: WAIS-III, BADS, WCST, Colour Trails, Trail Making A and B, BVRT, California Verbal Learning Test (spanish version: TAVEC). Variables was summarized determining Z values and principal components. It was constructed one Logistic Regression Model to determinate the better predicting model of drug use state.

The resultant model included two predictors: WAIS Perceptive Organization Index and Trails Component. The prediction formula for Ln Odd Ratio of drug abuse/dependence group is: -13.83 + 1.09 (Trails Z score) + 0.16 (WAIS Perceptive Organization Index). It explains for 63% of the variance (p = 0.001). The ROC curve for using in diagnose was constructed.

Neuropsychological diagnosis can contribute to the differential diagnosis of schizophrenia in dual pathology. The better functioning in visual-spatial tasks increases the probability of psychosis related with drugs use.