Breakthrough PainAdministration, BuccalFentanylAnalgesics, OpioidLabor PainPainReceptors, Opioid, muPain MeasurementAnalgesia, EpiduralReceptors, OpioidReceptors, Opioid, deltaReceptors, Opioid, kappaDouble-Blind MethodNeoplasmsChronic DiseaseOpioid PeptidesChronic PainPain ManagementPain ThresholdNarcotic AntagonistsPain, PostoperativePain, IntractableRadionuclide GeneratorsLow Back PainBack PainNaloxoneOpioid-Related DisordersMorphineAbdominal PainNeck PainNaltrexonePain PerceptionPelvic PainAcute PainEnkephalinsAnalgesicsFacial PainEnkephalin, Ala(2)-MePhe(4)-Gly(5)-