Chronic PainAnalgesicsAnalgesics, OpioidPainReceptors, Opioid, muReceptors, OpioidPain MeasurementPain ManagementReceptors, Opioid, deltaReceptors, Opioid, kappaPain ThresholdPain, IntractableAnalgesics, Non-NarcoticOpioid PeptidesPain, PostoperativeChronic DiseaseAnalgesiaMorphineNarcotic AntagonistsOpioid-Related DisordersAcute PainPain PerceptionNaloxoneBack PainNeuralgiaHyperalgesiaLow Back PainNeck PainPain ClinicsAbdominal PainFacial PainNaltrexoneTramadolNarcoticsDrug ToleranceBuprenorphinePelvic PainMusculoskeletal PainNociceptorsEnkephalinsEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Injections, SpinalFentanylNociceptive PainEndorphinsHydromorphoneOxycodoneFibromyalgiaAnesthetics, LocalShoulder PainEnkephalin, D-Penicillamine (2,5)-NociceptionAcetaminophenDynorphinsAnti-Inflammatory Agents, Non-SteroidalPain, ReferredTreatment OutcomeDose-Response Relationship, DrugCodeineSpinal CordRats, Sprague-DawleyDouble-Blind MethodQuestionnairesMethadonePhenacetinbeta-EndorphinHydrocodoneMorphinansEnkephalin, LeucineDextropropoxypheneComplex Regional Pain SyndromesBupivacaineMeperidineCyclohexanecarboxylic AcidsTime FactorsPrescription Drug MisuseArthralgiaEnkephalin, Leucine-2-AlanineOxymorphonePentazocineAcupuncture AnalgesiaNerve BlockPrescription DrugsCarrageenanPosterior Horn CellsBehavior, AnimalAnalgesia, Patient-ControlledAnalgesia, EpiduralBenzeneacetamidesButorphanolMyofascial Pain SyndromesTranscutaneous Electric Nerve StimulationCentral Nervous System SensitizationSpinal NervesDiprenorphineDisability EvaluationLidocaineDisease Models, AnimalProspective StudiesTemporomandibular Joint Disorders