Opioid Analgesia
Pharmacology
- codone opioids undergo Phase 1 metabolism by CYP enzymes to their corresponding morphone drugs. For example, oxycodone becomes oxymorphone and hydrocodone becomes hydromorphone.
Opioid Equianalgesic Conversion Tables
| Opioid | Oral Morphine Mg Equivalent (MME) | Initial Adult Oral Dose |
|---|---|---|
| Morphine | 1 | 5-20 mg q4h |
| Hydromorphone | 5 | 1-4 mg po q4h |
| Codeine | 0.15 | 15-60mg q4h |
| Oxycodone | 1.5 | 5-10 mg q4h |
Notable Opioids
Morphine
Initial dose: 0.05 mg/kg IV
The gold standard for comparing all other opioids' effectiveness and adverse reactions. Generally considered the first-choice opioid.
M3G metabolite has neuroexcitatory properties which contributes to toxicity. M6G active metabolite is renally cleared, and theoretically contributes to respiratory depression.
Hydromorphone
Onset of IV action is 5-20 minutes due to its high lipid solubility.
Consider this more strongly in renal disease as relatively lower concentration of renally cleared metabolites is produced. However, its HM3G metabolite is 2.5x more neuroexcitatory than M3G. It does not have an analgesically active metabolite unlike morphine.
Internists tend to prefer this, although dosing errors are more common due to the small quantities this drug is dispensed in relative to morphine.
Codeine
A significant number of people are either poor or rapid metabolizers of codeine (to its morphine metabolite), which can lead to toxicity (in both groups) and/or poor treatment of their pain.
Tramadol
Risk for serotonin syndrome and other toxicity due to inhibition of norepinephrine and serotonin reuptake. Avoid with MAO inhibitors, SSRIs, SNRIs.
Fentanyl
Methadone
Opioid Neurotoxicity
Signs:
- myoclonus
- decreased respiratory rate
- miosis
References
- Canadian Guideline for Opioid Use for Pain — Appendix B-8: Opioid Conversion and Brand Availability in Canada (mcmaster.ca)
- Chronic Pain Management Toolkit - Conversion Table (aafp.org)
- Opioid Metabolism (nih.gov)
- Hydromorphone - Journal of Pain and Symptom Management (jpsmjournal.com)