A conservative estimate suggests that over 10.2 million people are held in penal institutions throughout the world, which equates to a world prison population of 144 per 100,000 (Walmsley, 2013). Internationally, drug dependence is common amongst prison populations and many are dependent upon heroin. A systematic review of international prevalence studies of prison populations for drug dependence found prevalence rates for drug dependence to vary from 10% to 48% in male prisoners and 30% to 60% in female prisoners (Fazel et al., 2006). (Source: Drug and Alcohol Dependence)