Street DrugsHeroin DependenceSyringesCrack CocaineNeedle SharingNeedle-Exchange ProgramsSubstance Abuse, IntravenousHeavy IonsDopamine Uptake InhibitorsRisk-TakingDrug UsersSelf AdministrationHepatitis CNarcoticsSubstance-Related DisordersBehavior, AddictiveFar EastSubstance Abuse DetectionConditioning, OperantAmphetamine-Related DisordersNucleus AccumbensOpioid-Related DisordersHomeless Youth