AnalgesicsAnalgesics, OpioidReceptors, Opioid, muReceptors, OpioidReceptors, Opioid, deltaReceptors, Opioid, kappaOpioid PeptidesAnalgesics, Non-NarcoticMorphineNarcotic AntagonistsPainAnalgesiaPain MeasurementNaloxoneOpioid-Related DisordersNaltrexonePain, PostoperativeEnkephalinsTramadolNarcoticsEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Drug ToleranceBuprenorphineEndorphinsEnkephalin, D-Penicillamine (2,5)-FentanylDelayed DiagnosisPhenacetinDynorphinsAcetaminophenPrenatal DiagnosisPain ThresholdHydromorphoneEarly DiagnosisEnkephalin, LeucineMorphinansDextropropoxypheneCodeineInjections, SpinalSensitivity and SpecificityAnti-Inflammatory Agents, Non-SteroidalEnkephalin, Leucine-2-AlanineHyperalgesiaPain Managementbeta-EndorphinDose-Response Relationship, DrugMeperidinePentazocineMethadonePain, IntractableBenzeneacetamidesRetrospective StudiesDiprenorphineDiagnosisChronic PainTime FactorsAnesthetics, LocalHydrocodoneButorphanolDouble-Blind MethodOxymorphoneMorphine DerivativesAnalgesia, EpiduralTreatment OutcomeDipyroneLevorphanolAnalgesia, Patient-ControlledBenzomorphansProspective StudiesBupivacaineRats, Sprague-DawleyOxycodoneNociceptorsAlfentanilPrescription DrugsCyclazocineSubstance Withdrawal SyndromeNociceptionNalbuphineNeuralgiaHeroinPyrrolidinesTolmetinCarrageenanKetoprofenSufentanilMeptazinolKetorolacAcupuncture AnalgesiaDiagnostic ErrorsMorphine DependenceChronic DiseaseAcute PainPregnancyDrug OverdoseKidney Papillary NecrosisSpinal CordNalorphineTomography, X-Ray ComputedNociceptive Pain