The ability of addictive drugs to produce physical dependence is also studied at the ARU. The discomfort experienced during withdrawal of some drugs (e.g., heroin) has long been presumed a factor in continued drug intake and addiction. Physical dependence studies investigate the motivational properties of somatic withdrawal reactions. Although physical dependence is not a necessary condition for drug addiction, it may contribute to the total reinforcing impact of some drugs. Therefore, limited studies investigating physical dependence are conducted at the ARU.