Background and Aim: Patients with poly-substance use disorders and their personality disorder features have recently attracted attention among researchers (e.g., Preti et al., 2011). This population may have some personality features that draw them into seeking more gains and personal advantages regardless of the cost to others. Personality disorder features, especially those known as cluster B including antisocial and narcissistic personality disorders, are great measures that predict these behaviors, as they mainly involve how one makes a balance between one’s and others' benefits. Therefore, in the current study we were interested in comparing patients with mono- and polysubstance use disorders regarding these types of personality disorder features. Methods: Twenty five outpatient addicts under Methadone Maintenance Treatment (MMT) were selected as a convenient sample. They were divided into a couple of groups, mono-substance users (12 opium users) and poly-substance users (13 users of more than one type drug). The two groups were homogeneous in terms of age and education as there were no significant differences between the two. All patients filled out a 16-item Narcissistic Disorder Inventory (NPI-16, Ames et al., 2006) and antisocial (psychopath) questionnaire (Levenson et al., 1995), shown good psychometric properties in our previous studies. Results: An independent t-test showed a significant difference among the two groups in terms of antisocial personality disorder features (t23=2.1; p < .05, Levene's Test for Equality of Variances assumed). Both groups were very low on narcissistic disorder features, with no significant differences between the two groups. Further analysis of the data from both groups using Pearson correlation showed a significant negative relationship between age and narcissistic personality disorder (r = -.43; p < .05) and a significant negative relationship between education and antisocial personality disorder (r = -.49; p < .01). No oth
