AnalgesiaAnalgesia, EpiduralAnalgesia, ObstetricalAnalgesics, OpioidAnalgesia, Patient-ControlledMorphinePain, PostoperativePain MeasurementFentanylAnalgesicsPainBupivacaineAnesthetics, LocalNaloxoneAcupuncture AnalgesiaNarcoticsSufentanilMeperidineReceptors, OpioidNarcotic AntagonistsOpioid PeptidesAnesthesia, EpiduralInjections, SpinalOpioid-Related DisordersOxycodoneHydromorphoneLabor PainTramadolEnkephalin, MethionineEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Nerve BlockDrug ToleranceAnesthesia, ObstetricalLabor, ObstetricAlfentanilReceptors, Opioid, kappaAnalgesics, Non-NarcoticEnkephalinsEndorphinsDouble-Blind MethodAnesthesia, SpinalPain ThresholdAmidesReceptors, Opioid, deltaPain, IntractablePain ManagementNaltrexonePostoperative Nausea and VomitingBuprenorphineCesarean SectionEnkephalin, D-Penicillamine (2,5)-OxymorphoneMorphine DerivativesHypnotics and SedativesChronic Painbeta-EndorphinCodeineConscious SedationAnesthesia, GeneralKetorolacClonidineNalbuphineInterpleural AnalgesiaPeriaqueductal GrayButorphanolEnkephalin, LeucineLidocaineHyperalgesiaDose-Response Relationship, DrugKetamineReceptors, Opioid, muDynorphinsHeroinPruritusNociceptive PainLabor Stage, FirstPrescription DrugsPregnancyPiperidinesAnesthetics, CombinedAnesthesia, CaudalAdjuvants, AnesthesiaElectroacupunctureAnesthesia and AnalgesiaMethadoneBenzeneacetamidesFemoral NerveHydrocodonePirinitramideNociceptorsSpinal CordRats, Sprague-DawleyProspective StudiesNitrous OxideAnesthesia, ConductionPrescription Drug MisusePentazocineAcute PainThoracotomyEtorphine