AnalgesicsAnalgesics, OpioidReceptors, Opioid, muReceptors, OpioidReceptors, Opioid, deltaReceptors, Opioid, kappaOpioid PeptidesAnalgesics, Non-NarcoticMorphineNarcotic AntagonistsPainAnalgesiaPain MeasurementNaloxoneOpioid-Related DisordersNaltrexonePain, PostoperativeEnkephalinsTramadolNarcoticsEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Drug ToleranceBuprenorphineEndorphinsFentanylEnkephalin, D-Penicillamine (2,5)-DynorphinsAntimitotic AgentsAcetaminophenTime FactorsPain ThresholdHydromorphoneEnkephalin, LeucineMorphinansDose-Response Relationship, DrugDextropropoxypheneCodeineInjections, SpinalAnti-Inflammatory Agents, Non-SteroidalEnkephalin, Leucine-2-AlanineHyperalgesiabeta-EndorphinPain ManagementMeperidineMethadoneAnimal Assisted TherapyAnesthetics, LocalDouble-Blind MethodPain, IntractableBenzeneacetamidesDiprenorphineChronic PainButorphanolHydrocodoneRats, Sprague-DawleyBupivacaineOxymorphoneAnalgesia, EpiduralMorphine DerivativesDipyroneAnalgesia, Patient-ControlledLevorphanolBenzomorphansNociceptorsOxycodoneTreatment OutcomeAlfentanilSubstance Withdrawal SyndromeNociceptionCyclazocineNalbuphinePrescription DrugsPyrrolidinesHeroinNeuralgiaSufentanilKetoprofenTolmetinCarrageenanAcupuncture AnalgesiaKetorolacMeptazinolMorphine DependenceBehavior, AnimalSpinal CordProspective StudiesNalorphineDrug InteractionsAcute PainNociceptive PainKidney Papillary NecrosisDrug OverdosePeriaqueductal GrayCyclohexanecarboxylic AcidsAnalgesia, ObstetricalIbuprofenNefopamPrescription Drug MisuseOpiate Substitution TreatmentDrug Administration Schedule