PrevalenceReceptors, Opioid, muReceptors, OpioidAnalgesics, OpioidReceptors, Opioid, deltaReceptors, Opioid, kappaOpioid PeptidesNarcotic AntagonistsNaloxoneOpioid-Related DisordersNaltrexoneEnkephalinsEnkephalin, Ala(2)-MePhe(4)-Gly(5)-MorphineNarcoticsCross-Sectional StudiesEnkephalin, D-Penicillamine (2,5)-EndorphinsRisk FactorsDynorphinsBuprenorphineEnkephalin, LeucinePainEnkephalin, Leucine-2-AlanineDiprenorphinebeta-EndorphinAnalgesicsAge DistributionBenzeneacetamidesAge FactorsPain MeasurementMethadoneSex DistributionDrug ToleranceMorphinansFentanylSex FactorsBrazilUnited StatesQuestionnairesHealth SurveysHydromorphoneBenzomorphansRural PopulationSubstance Withdrawal SyndromePyrrolidinesAnalgesiaSocioeconomic FactorsUrban PopulationOpiate Substitution TreatmentLogistic ModelsRetrospective StudiesChronic PainIndiaChinaCohort StudiesChronic DiseaseSmokingPrescription Drug MisuseComorbidityHIV InfectionsPrescription DrugsIncidenceTime FactorsHeroinDose-Response Relationship, DrugLevorphanolPopulation SurveillanceProspective StudiesInfant, NewbornHeroin DependenceMorphine DependenceOdds RatioObesityButorphanolPregnancySubstance-Related DisordersEpidemiologic MethodsDiterpenes, ClerodaneMass ScreeningSeverity of Illness IndexRural HealthPolymerase Chain ReactionOxymorphoneGenotypePain, IntractableEthylketocyclazocineRats, Sprague-DawleyChi-Square DistributionMorphine DerivativesInjections, SpinalBody Mass IndexDrug OverdoseFecesOligopeptidesIranDiabetes MellitusSpainJapanAnalgesics, Non-Narcotic