Different studies have used different criteria for defining specific addictions or different criteria for the definition of controls. For example, in our studies we have analyzed heroin addicts that have met the stringent criteria of entering into methadone maintenance treatment (MMT) (at least one year of multiple daily use), while other studies rely on DSM-IV criteria that may be less stringent. Several studies use the more general phenotypes that are based on the concept of common drug use disorders (DUD) liability (Vanyukov et al. 2003), including substance dependence (SD), substance use disorder (SUD), and illicit drug dependence.
