Substance dependence (SD) is a chronic psychiatric illness [Leshner, 1997; McLellan et al., 2000]. This illness has led to substantial injury, loss of life, and disability in societies worldwide. Developing effective treatments for SD is particularly crucial because of the exceptionally high rate of relapse [McGovern et al., 2005] and the tremendous disease burden to society. Dependence on multiple substances, which is frequently observed clinically, complicates the development of effective treatments. Multiple substance use is common among alcohol and drug users and the average percentage of individuals using multiple substances is around 56% [SAMSHA, 2009]. Individuals with comorbid DSM-IV alcohol and drug use disorders outnumber patients with a drug use or alcohol use disorder alone [Stinson et al., 2005] among treatment seekers. This treatment seeking pattern may be a reflection of disease severity and the damage that multiple substance dependence(MSD) inflicts on an individual’s health and well being. Indeed, MSD patients are often severely affected.
