PainAnalgesics, OpioidChronic PainPain ManagementPain ThresholdPain MeasurementPain, PostoperativePain, IntractableLow Back PainBack PainNarcoticsMorphineOpioid-Related DisordersAbdominal PainNeck PainAcute PainAnalgesicsPelvic PainNaloxonePain PerceptionOxycodoneNarcotic AntagonistsFacial PainOpioid PeptidesReceptors, OpioidEnkephalin, MethionineFentanylPain, ReferredEnkephalin, Ala(2)-MePhe(4)-Gly(5)-AnalgesiaShoulder PainNociceptive PainMusculoskeletal PainReceptors, Opioid, kappaEndorphinsEnkephalinsHydromorphoneNeuralgiaReceptors, Opioid, deltaHyperalgesiaNaltrexoneChronic DiseasePrescription DrugsAlfentanilEnkephalin, D-Penicillamine (2,5)-OxymorphoneInjections, SpinalBuprenorphineDynorphinsDrug Tolerancebeta-EndorphinTramadolCodeinePrescription Drug MisuseEnkephalin, LeucineHydrocodoneSufentanilNociceptorsMethadoneAnalgesics, Non-NarcoticAnesthetics, LocalBenzeneacetamidesTreatment OutcomeMyofascial Pain SyndromesMeperidineComplex Regional Pain SyndromesVisceral PainAnalgesia, EpiduralDose-Response Relationship, DrugMorphine DerivativesDouble-Blind MethodEnkephalin, Leucine-2-AlanineArthralgiaRats, Sprague-DawleyHeroinSpinal CordOpiate AlkaloidsPeriaqueductal GrayLabor PainEtorphineQuestionnairesPyrrolidinesAnalgesia, Patient-ControlledPalliative CareArkansasTime FactorsHypnotics and SedativesBreakthrough PainDrug PrescriptionsEuphoriaProspective StudiesSubstance-Related DisordersDiprenorphineDisability EvaluationNerve BlockBupivacaineSeverity of Illness IndexDrug and Narcotic ControlFibromyalgiaLidocaine