Analgesics, OpioidNarcoticsOpioid-Related DisordersMorphineNaloxoneOxycodoneEnkephalin, MethionineNarcotic AntagonistsReceptors, OpioidOpioid PeptidesPainEnkephalin, Ala(2)-MePhe(4)-Gly(5)-FentanylReceptors, Opioid, kappaPrescription DrugsAnalgesicsEnkephalinsEndorphinsHydromorphonePain MeasurementReceptors, Opioid, deltaChronic PainNaltrexoneEnkephalin, D-Penicillamine (2,5)-AlfentanilOxymorphoneAnalgesiaDrug TolerancePain, IntractableBuprenorphineDynorphinsEnkephalin, Leucinebeta-EndorphinDrug InteractionsDose-Response Relationship, DrugPrescription Drug MisuseCodeineSufentanilMorphine DerivativesHydrocodoneBenzeneacetamidesDrug PrescriptionsMethadoneInjections, SpinalDrug and Narcotic ControlTramadolHypnotics and SedativesSubstance-Related DisordersEnkephalin, Leucine-2-AlanineOpiate AlkaloidsMeperidineEtorphineDrug OverdoseHeroinPharmaceutical PreparationsPyrrolidinesPsychotropic DrugsBenzodiazepinesAnalgesics, Non-NarcoticArkansasRats, Sprague-DawleyPain ManagementStreet DrugsNociceptive PainEuphoriaChronic DiseaseDesigner DrugsPeriaqueductal GrayPain, PostoperativeSubstance Abuse DetectionDiprenorphineAnalgesia, EpiduralPiperidinesDrug UtilizationDrug-Related Side Effects and Adverse ReactionsNeoplasmsAnticonvulsantsPsychopharmacologyLevorphanolHyperalgesiaTranquilizing AgentsPain ThresholdAcute PainTreatment OutcomeOrphan Drug ProductionTime FactorsLegislation, DrugSubstance Withdrawal SyndromeDrug ApprovalOpiate Substitution TreatmentDrug Therapy, CombinationAnti-Inflammatory Agents, Non-SteroidalDouble-Blind MethodMethadyl AcetateDrug SynergismDrug CombinationsNalbuphineD-Ala(2),MePhe(4),Met(0)-ol-enkephalinAntipsychotic AgentsBreakthrough Pain