Table 1.

Summary of the primary and secondary metabolic pathways for commonly prescribed opioids.

OpioidPrimary metabolic pathwaySecondary metabolic pathway(s)
CodeineCodeine-6-glucuronide via UGT2B7Norcodeine via CYP3A4
Morphine via CYP2D6
HydrocodoneNorhydrocodone via CYP3A4Hydromorphone via CYP2D6
Dihydrocodeine via 6-keto-reductase
TramadolO-Desmethyltramadol via CYP2D6N-Desmethyltramadol via CYP2B6 and CYP3A4
MorphineMorphine-3-glucuronide via UGT2B7Morphine-6-glucuronide via UGT2B7
Normorphine via CYP3A4
HydromorphoneHydromorphone-3-glucuronide via UGT2B7Hydromorphone-6-glucuronide via UGT2B7
OxycodoneNoroxycodone via CYP3A4Oxymorphone via CYP2D6
OxymorphoneOxymorphone-3-glucuronide via UGT2B76-OH-Oxymorphone via unknown enzyme
FentanylNorfentanyl via CYP3A4Hydroxyfentanyl via unknown enzyme
MethadoneEDDP via CYP2B6Minor metabolites via CYP3A4, CYP2D6, CYP2C9, CYP2C19
BuprenorphineNorbuprenorphine via CYP3A4Buprenorphine-3-glucuronide via UGT2B7
Naltrexone6β-Naltrexol via dihydrodiol dehydrogenase2-Hydroxy-3-methoxynaltrexone
NaloxoneNaloxone-3-glucuronide via UGT1A8 and UGT2B7Minor metabolites