Analgesics, OpioidAnalgesicsReceptors, Opioid, muPainReceptors, OpioidOpioid-Related DisordersOxycodoneReceptors, Opioid, deltaTramadolMorphineChronic PainPain MeasurementReceptors, Opioid, kappaOxymorphoneAnalgesics, Non-NarcoticPain ManagementNarcotic AntagonistsPain, PostoperativeMeptazinolHydrocodoneAnalgesiaNaloxoneMeperidinePain ThresholdFentanylAcute PainOpioid PeptidesHydromorphoneDextropropoxypheneDrug ToleranceDrug and Narcotic ControlPain, IntractableButorphanolNaltrexoneCodeineNarcoticsPrescription Drug MisusePrescription DrugsBuprenorphinePentazocineMorphinansEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Drug PrescriptionsAcetaminophenMorphine DerivativesMethadoneNalbuphineLow Back PainBack PainInjections, SpinalDose-Response Relationship, DrugAlfentanilInappropriate PrescribingAbdominal PainAnti-Inflammatory Agents, Non-SteroidalCyclohexanecarboxylic AcidsNeck PainPain PerceptionUncompensated CareDouble-Blind MethodEnkephalinsPharmaceutical ServicesFacial PainPelvic PainEndorphinsHyperalgesiaChronic DiseaseEnkephalin, D-Penicillamine (2,5)-PoisoningDrug InteractionsRats, Sprague-DawleyPain, ReferredNeuralgiaDynorphinsNociceptive PainPhenacetinAnesthetics, LocalShoulder PainEnkephalin, LeucineHot TemperatureMusculoskeletal PainUnited StatesNociceptorsQuestionnairesbeta-EndorphinEnkephalin, Leucine-2-AlanineSubstance-Related DisordersRetrospective StudiesAnalgesia, Patient-ControlledAnalgesia, EpiduralBupivacaineBenzeneacetamidesNociceptionDiprenorphineDipyroneNeoplasmsAcupuncture AnalgesiaTreatment OutcomeLevorphanolCanada