Breakthrough PainMorphineAnalgesics, OpioidAdministration, BuccalPainFentanylLabor PainPain MeasurementMorphine DependenceAnalgesicsDrug ToleranceNarcoticsPain ManagementReceptors, Opioid, muAnalgesia, EpiduralChronic PainPain, PostoperativeMorphine DerivativesSubstance Withdrawal SyndromeNaloxoneNarcotic AntagonistsPain, IntractableDouble-Blind MethodPrescription Drug MisuseNeoplasmsLevallorphanEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Locus CoeruleusChronic DiseaseReceptors, Opioid, deltaNaltrexonePeriaqueductal GrayArthralgiaTime FactorsTreatment OutcomeEnkephalinsMedication ErrorsReceptors, OpioidMedication AdherenceAnalgesiaRats, Sprague-DawleyNeuralgiaPalliative CarePain ThresholdQuestionnairesDose-Response Relationship, DrugProspective StudiesAnti-Inflammatory Agents, Non-SteroidalNucleus AccumbensHyperalgesiaDrug Administration ScheduleOpioid-Related DisordersRadionuclide GeneratorsRetrospective StudiesOpioid PeptidesEnkephalin, MethionineLow Back PainBack PainSeverity of Illness IndexBehavior, AnimalPatient SatisfactionQuality of LifeAbdominal PainFollow-Up StudiesNeck PainComorbidityMedication ReconciliationMedication SystemsSpinal CordPain PerceptionAdenylate CyclaseMedication Systems, HospitalCohort StudiesPelvic PainFacial PainArrestinsInjections, SpinalBrainMotor ActivityAcute PainG Protein-Coupled Inwardly-Rectifying Potassium ChannelsCyclic AMP-Dependent Protein KinasesNeurons