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)-NarcoticsEnkephalin, D-Penicillamine (2,5)-EndorphinsDynorphinsBuprenorphineEnkephalin, LeucineEnkephalin, Leucine-2-AlaninePainDiprenorphinebeta-EndorphinAnalgesicsBenzeneacetamidesDrug ToleranceFentanylTreatment OutcomeMorphinansMethadoneHydromorphoneBenzomorphansDose-Response Relationship, DrugPyrrolidinesAnalgesiaRetrospective StudiesTime FactorsSubstance Withdrawal SyndromeOpiate Substitution TreatmentHeroinMedian EminenceFollow-Up StudiesChronic PainPrescription Drug Misuse