AnalgesicsAnalgesics, OpioidPrevalenceReceptors, Opioid, muReceptors, OpioidReceptors, Opioid, deltaReceptors, Opioid, kappaOpioid PeptidesAnalgesics, Non-NarcoticMorphineNarcotic AntagonistsPainAnalgesiaPain MeasurementNaloxoneOpioid-Related DisordersNaltrexonePain, PostoperativeEnkephalinsNarcoticsTramadolEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Drug ToleranceBuprenorphineEndorphinsEnkephalin, D-Penicillamine (2,5)-FentanylCross-Sectional StudiesDynorphinsPhenacetinRisk FactorsAcetaminophenPain ThresholdHydromorphoneEnkephalin, LeucineDextropropoxypheneMorphinansCodeineInjections, SpinalEnkephalin, Leucine-2-AlanineAnti-Inflammatory Agents, Non-SteroidalHyperalgesiaPain Managementbeta-EndorphinMethadoneDose-Response Relationship, DrugMeperidinePentazocinePain, IntractableBenzeneacetamidesChronic PainDiprenorphineAge FactorsAnesthetics, LocalButorphanolHydrocodoneAge DistributionDouble-Blind MethodQuestionnairesOxymorphoneMorphine DerivativesSex FactorsDipyroneAnalgesia, EpiduralLevorphanolUnited StatesAnalgesia, Patient-ControlledSex DistributionTime FactorsPrescription DrugsBenzomorphansBrazilRats, Sprague-DawleyBupivacaineOxycodoneChronic DiseaseProspective StudiesSubstance Withdrawal SyndromeNociceptorsAlfentanilCyclazocineHealth SurveysNociceptionNalbuphineNeuralgiaPyrrolidinesHeroinRetrospective StudiesTolmetinCarrageenanKetoprofenSufentanilMeptazinolKetorolacAcupuncture AnalgesiaRural PopulationLogistic ModelsMorphine DependencePrescription Drug MisuseUrban Population