Analgesics, OpioidReceptors, Opioid, muReceptors, OpioidOpioid-Related DisordersMorphineNarcotic AntagonistsReceptors, Opioid, deltaNaloxoneNarcoticsBuprenorphineReceptors, Opioid, kappaDrug ToleranceOpioid PeptidesPainNaltrexoneEnkephalinsEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Enkephalin, D-Penicillamine (2,5)-EndorphinsDynorphinsEnkephalin, LeucineEnkephalin, Leucine-2-AlanineDiprenorphineDisabled Personsbeta-EndorphinBenzeneacetamidesAnalgesicsPain MeasurementMethadoneMorphinansFentanylHydromorphoneBenzomorphansPyrrolidinesSubstance Withdrawal SyndromeAnalgesiaOpiate Substitution TreatmentHeroinUnited StatesChronic PainDose-Response Relationship, DrugPrescription Drug MisuseLevorphanolPrescription DrugsMorphine DependenceHomeless PersonsButorphanolHeroin DependenceDiterpenes, ClerodaneOxymorphoneEthylketocyclazocineMorphine DerivativesDrug OverdoseInjections, SpinalRats, Sprague-DawleyOligopeptidesPain, IntractableRisk FactorsMentally Ill PersonsBenzylidene CompoundsAnalgesics, Non-NarcoticVisually Impaired PersonsPain ThresholdCyclazocineTime FactorsMentally Disabled PersonsHIV InfectionsPeriaqueductal GrayD-Ala(2),MePhe(4),Met(0)-ol-enkephalinHyperalgesiaChronic DiseaseAlfentanilNalbuphineHydrocodoneTramadolPrevalenceDrug InteractionsGuanosine 5'-O-(3-Thiotriphosphate)Substance-Related DisordersPain ManagementAge FactorsQuestionnairesLoperamideRadioligand AssaySufentanilOpiate AlkaloidsIncidenceCohort StudiesPain, PostoperativeBrainMethadyl AcetateCodeineAzocinesSpinal CordNociceptorsSex FactorsCross-Sectional StudiesAnalysis of VariancePiperidinesLigands