Street DrugsCocaineCocaine-Related DisordersHeroin DependenceSyringesCrack CocaineNeedle SharingNeedle-Exchange ProgramsSubstance Abuse, IntravenousBaltimoreDopamine Uptake InhibitorsRisk-TakingDrug UsersSelf AdministrationHepatitis CNarcoticsSubstance-Related DisordersBehavior, AddictiveArkansasSubstance Abuse DetectionConditioning, OperantAmphetamine-Related DisordersNucleus AccumbensOpioid-Related DisordersHomeless Youth