Chronic PainPainPain ManagementAnalgesics, OpioidPain MeasurementPain, IntractablePain ThresholdChronic DiseasePain, PostoperativeAnalgesicsAcute PainOpioid-Related DisordersBack PainPain PerceptionLow Back PainNeuralgiaPain ClinicsNeck PainMorphineOxycodoneAbdominal PainHyperalgesiaNarcoticsFacial PainAnalgesiaPelvic PainMusculoskeletal PainNarcotic AntagonistsNaloxoneReceptors, OpioidOpioid PeptidesNociceptive PainFentanylEnkephalin, MethionineShoulder PainFibromyalgiaNociceptorsHydromorphonePain, ReferredEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Injections, SpinalQuestionnairesEndorphinsPrescription Drug MisuseComplex Regional Pain SyndromesEnkephalinsBuprenorphinePrescription DrugsReceptors, Opioid, kappaDrug ToleranceSpinal CordAnalgesics, Non-NarcoticReceptors, Opioid, deltaOxymorphoneHydrocodoneNaltrexoneArthralgiaTreatment OutcomeAnesthetics, LocalMyofascial Pain SyndromesTramadolNociceptionbeta-EndorphinMethadoneDynorphinsRats, Sprague-DawleyCentral Nervous System SensitizationCodeineDisability EvaluationPosterior Horn CellsTemporomandibular Joint DisordersAlfentanilEnkephalin, D-Penicillamine (2,5)-Quality of LifeDepressionSeverity of Illness IndexDose-Response Relationship, DrugSpinal NervesHernia, InguinalTime FactorsDouble-Blind MethodAdaptation, PsychologicalPeripheral Nervous System DiseasesVisceral PainDrug and Narcotic ControlEnkephalin, LeucineSubstance Abuse DetectionNerve BlockGanglia, SpinalSufentanilPeriaqueductal GrayCyclohexanecarboxylic AcidsCarisoprodolActivities of Daily LivingProspective StudiesMusculoskeletal DiseasesPalliative CareTrigeminal NeuralgiaCross-Sectional StudiesSubstance-Related Disorders