Pain, PostoperativePostoperative ComplicationsTreatment OutcomeAnalgesics, OpioidPostoperative PeriodPainRetrospective StudiesFollow-Up StudiesChronic PainProspective StudiesPain ManagementNarcoticsPain MeasurementOpioid-Related DisordersMorphinePain, IntractablePain ThresholdNaloxoneNarcotic AntagonistsEnkephalin, MethionineOxycodoneOpioid PeptidesReceptors, OpioidFentanylAnalgesicsEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Back PainLow Back PainReceptors, Opioid, kappaAcute PainEnkephalinsEndorphinsAnalgesiaAbdominal PainHydromorphoneNeck PainPost and Core TechniqueReceptors, Opioid, deltaPelvic PainNaltrexonePrescription DrugsPain PerceptionEnkephalin, D-Penicillamine (2,5)-AlfentanilOxymorphoneFacial PainNociceptive PainBuprenorphineDynorphinsPain, Referredbeta-EndorphinDrug ToleranceInjections, SpinalOperative TimeEnkephalin, LeucinePrescription Drug MisuseCodeineNeuralgiaChronic DiseaseSufentanilTramadolHydrocodoneHyperalgesiaTime FactorsShoulder PainMethadoneBenzeneacetamidesMusculoskeletal PainMeperidineEnkephalin, Leucine-2-AlanineMorphine DerivativesDose-Response Relationship, DrugAnalgesia, EpiduralAnalgesics, Non-NarcoticLaparoscopyOpiate AlkaloidsHeroinEtorphinePyrrolidinesAnesthetics, LocalNociceptorsRats, Sprague-DawleyPeriaqueductal GraySurgical Procedures, OperativeArkansasPalliative CareDouble-Blind MethodEuphoriaDrug PrescriptionsHypnotics and SedativesSpinal CordSubstance-Related DisordersDiprenorphineAnalgesia, Patient-ControlledBreakthrough PainDentistry, OperativeLevorphanolDrug and Narcotic ControlDrug OverdoseOpiate Substitution Treatment