So far, research on cognitive functions in multiple drug abusers with a very early age of onset has been only tentative. Furthermore, outcome research for therapy or rehabilitation is urgently needed in subgroups of this heterogeneous cohort of drug abusers.

Different cognitive domains (i.e. intelligence and attentiveness) of N = 750 patients were tested in a rehabilitation hospital for multiple drug abuse and comorbid mental disorder. The test results were correlated with the prominent drug cluster of the patients over the last 10 years. Calculated were also the impact of age of onset and mental disorder on the attentional system.

Drug clustering showed different effects on attentional functions. Morphine was hardly associated with serious deficits. Cannabis showed effects in the domains of multimodal stimulus processing (i.e. divided attention) and visual attention (visual scanning) with respect to cumulative lifetime dosage and the duration of use. Cumulative use of alcohol also had a negative effect on the attentional system, particularly on frontal executive functions (i.e. flexibility and working memory). The effect of amphetamines, methamphetamines and cocaine varied depending on which other drugs were used. Comorbidity (e.g., schizophrenia or developmental disorders) also had significant effects on the test results.

Specific attentional dysfunctions can lead to diminished behaviour control and impairments in professional abilities (e.g., ability to drive or operate a machine). The results of our study are important for diagnosis and planning therapy or rehabilitation especially in younger patients.