Analgesics, OpioidNarcoticsOpioid-Related DisordersMorphineNaloxoneEnkephalin, MethionineNarcotic AntagonistsReceptors, OpioidOpioid PeptidesOxycodonePainEnkephalin, Ala(2)-MePhe(4)-Gly(5)-FentanylReceptors, Opioid, kappaEnkephalinsEndorphinsAnalgesicsHydromorphoneReceptors, Opioid, deltaChronic PainPain MeasurementNaltrexoneEnkephalin, D-Penicillamine (2,5)-Prescription DrugsAlfentanilOxymorphoneAnalgesiaPain, IntractableBuprenorphineDynorphinsEnkephalin, Leucinebeta-EndorphinDrug TolerancePrescription Drug MisuseCodeineSufentanilHydrocodoneBenzeneacetamidesMethadoneInjections, SpinalTramadolEnkephalin, Leucine-2-AlanineMorphine DerivativesOpiate AlkaloidsMeperidineEtorphineHeroinPyrrolidinesArkansasDose-Response Relationship, DrugPain ManagementNociceptive PainEuphoriaPeriaqueductal GrayPain, PostoperativeDrug PrescriptionsDiprenorphineAnalgesia, EpiduralSubstance-Related DisordersHypnotics and SedativesLevorphanolHyperalgesiaPain ThresholdAcute PainAnalgesics, Non-NarcoticDrug and Narcotic ControlChronic DiseaseDrug OverdoseOpiate Substitution TreatmentRats, Sprague-DawleyMethadyl AcetateNalbuphineD-Ala(2),MePhe(4),Met(0)-ol-enkephalinDrug InteractionsBreakthrough PainPiperidinesPain ClinicsBenzomorphansSubstance Abuse DetectionNeuralgiaMorphinansAnalgesia, Patient-ControlledPalliative CareMorphine DependenceSpinal CordSubstance Withdrawal SyndromeDrug UtilizationButorphanolLoperamideNociceptorsAnesthetics, LocalPruritusBenzodiazepinesPrescription Drug DiversionLocus CoeruleusStreet DrugsAdministration, CutaneousHeroin DependenceNeonatal Abstinence SyndromeDouble-Blind Method