Anesthesia, RectalAnalgesics, OpioidInfusions, IntralesionalPain, PostoperativeAnalgesia, Patient-ControlledMorphinePain MeasurementReceptors, Opioid, muReceptors, OpioidReceptors, Opioid, deltaReceptors, Opioid, kappaOpioid PeptidesNarcotic AntagonistsNaloxoneOpioid-Related DisordersNaltrexoneEnkephalinsEnkephalin, Ala(2)-MePhe(4)-Gly(5)-NarcoticsSynthetic BiologyEndorphinsEnkephalin, D-Penicillamine (2,5)-DynorphinsBuprenorphineEnkephalin, LeucineGenes, SyntheticEnkephalin, Leucine-2-AlaninePainDiprenorphinebeta-EndorphinBenzeneacetamidesAnalgesicsMolecular Sequence DataAmino Acid SequenceDrug ToleranceFentanylMorphinansOligopeptidesMethadoneHydromorphonePeptidesDose-Response Relationship, DrugPyrrolidinesBenzomorphansAnalgesiaSubstance Withdrawal SyndromeOpiate Substitution TreatmentHeroinChronic PainPeptide FragmentsPrescription Drug Misuse