Understanding opioid metabolism is critical in palliative care, where drug interactions, patient-specific factors, and side effect profiles can significantly impact safety and comfort. This module offers a practical overview of hepatic metabolism pathways, genetic variability, and implications for drug selection and dosing.
Most opioids are metabolized in the liver via the cytochrome P450 system, particularly CYP2D6 and CYP3A4 enzymes. Metabolism affects not just drug clearance, but also the formation of active or toxic metabolites—key in patients with organ dysfunction, polypharmacy, or unusual responses.
Common Metabolic Pathways
- CYP2D6: Codeine, hydrocodone, oxycodone, tramadol, methadone
- CYP3A4: Fentanyl, oxycodone, methadone, buprenorphine
- Glucuronidation (non-CYP pathway): Morphine, hydromorphone, oxymorphone
See resource below for visual representation of metabolic pathways.
Key Concepts
- Genetic Variability
- Poor/Null metabolizers (common in Caucasians, African-Americans): may have reduced effect for prodrugs or buildup of parent drug in active compounds
- Ultra-rapid metabolizers (Middle Eastern, North African): risk of toxicity, especially with codeine/tramadol
- Inflammation: downregulates enzyme activity, affecting clearance.
- Medication Interactions:
- CYP2D6 inhibitors: fluoxetine, paroxetine
- CYP3A4 inhibitors: fluconazole, ritonavir; inducers: rifampin, phenytoin
Clinical Pearls
- Caution with all opioids in all patients with hepatic dysfunction. Those with significant liver dysfunction will display prolongation of analgesic effect, making long-acting medication more unpredictable (longer time to steady state/accumulation).
- A patient’s report of “allergy” may be a metabolic or side effect issue—true opioid allergy (anaphylaxis) is rare.
When in doubt, start low and increase by 50% or lower for uncontrolled pain, or consult a pharmacist. Always take a detailed medication history—how patients have responded to medications in the past can matter.