Opioid Conversion Guide

Clinical Disclaimer: This page explains the mathematical method used by the calculator. It does not replace clinical judgement. Always consider patient factors, incomplete cross-tolerance, organ function, and local guidelines.

1 Primary Opioid → Oral Morphine Equivalent Dose (oMED)

The calculator first converts the primary regular opioid into an equivalent oral morphine dose.

Primary Opioid oMED (mg/day) = Primary Dose × Primary Ratio [1][2][3][4]

Primary Dose = total 24-hour dose of the chosen primary opioid (e.g. PO oxycodone).
Primary Ratio = conversion factor from that opioid to PO morphine (from the ratio table below) [1][2][3][4].

2 Add All Additional Opioids → Total oMED

The calculator then adds in any additional/PRN opioids you enter (up to 3), converting each to PO morphine and summing them.

Total oMED (mg/day) = Primary Opioid oMED + Σ (Additional Dose × Additional Ratio) [1][2][3][4]

Additional Dose = total 24-hour dose for each other opioid.
Additional Ratio = its conversion factor to PO morphine [1][2][3][4].

3 Convert Total oMED → Output Opioid

The total PO morphine is then converted into the target opioid and route you selected in the "Convert to opioid" box.

Output Dose = Total oMED Ratio for Output Opioid [1][2][3][4]

Ratio for Output Opioid = the same type of ratio used in Step 1, but for the output opioid (from the ratio table below) [1][2][3][4].

Clinical Warnings

⚠️ Incomplete Cross-Tolerance

When switching between different opioids (especially at high doses >100mg PO morphine/day), patients exhibit incomplete cross-tolerance. Reduce calculated equianalgesic dose by 25-50% of calculated dose and titrate carefully based on response over 24-48 hours [5][6][7][8][9].

⚠️ Poor Oral Absorption

Patients with poor oral absorption (nausea, vomiting, bowel obstruction, dysphagia, cachexia) require parenteral routes (SC/IV). Do NOT include all PRN doses in 24-hour totals for these conversions - use only regular doses. Oral bioavailability is unpredictable due to poor absorption [10][18][19][20].

⚠️ Opioid Overlap

Long-acting formulations require careful overlap planning (4-24 hours) during rotation to avoid gaps or overdose.

CSCI → Fentanyl Patch (6-12hr overlap)
• Current: CSCI morphine 60mg/24h
• Apply fentanyl patch 50μg/hr (fentanyl peaks at 12-24hrs)
• Continue running CSCI for 6-12hrs [21][22]
IR Morphine → MST Slow Release (4hr overlap)
• Current: IR morphine 10mg q4h (60mg/24h)
• Give 1st MST 60mg WITH last IR dose (overlap 4hrs)
• Stop IR after MST taken (MST starts working ~4hrs) [23][24]
⚠️ Opioid-Refractory Pain

High opioid requirements with poor response suggest neuropathic component or opioid rotation failure. Consider methadone rotation (NMDA antagonism), ketamine infusion, or adjuvants (gabapentinoids). Methadone rotation shows ≥30-50% pain reduction in refractory cases. Burst ketamine reverses opioid tolerance - 60-80% response rate in cancer pain [14][15][25].

⚠️ Total Pain Management

Pain has physical, psychological, social, and spiritual dimensions ("total pain"). High opioid needs often indicate "total pain" requiring multidisciplinary team (psychology, physiotherapy, social work, spiritual care) alongside pharmacotherapy [16][17].

Default Ratios Used in the Calculator [1][2][3][4]

Opioid Route Ratio to PO Morphine
MorphinePO1.0
MorphineSC/IV2.0
OxycodonePO1.5
OxycodoneSC/IV3.0
TramadolPO0.2
TramadolIV0.2
CodeinePO0.1
DihydrocodeinePO0.1
Fentanyl PatchTD2.4
FentanylSC/IV0.1

Worked Example

Example inputs:

Step 1 – Primary PO Morphine:
40 mg × 1.5 = 60 mg PO morphine [1][2][3][4]

Step 2 – Total PO Morphine:
Additional SC morphine: 20 mg × 2.0 = 40 mg PO morphine [1][2][3][4]
Total PO morphine = 60 + 40 = 100 mg / day

Step 3 – Convert to SC oxycodone:
Ratio for SC oxycodone = 3.0 [1][2][3][4]
Output Dose = 100 3.0 ≈ 33.3 mg SC oxycodone / day

References

  1. Faculty of Pain Medicine, ANZCA. (2021). Opioid dose equivalence.
  2. Wilcock, A., Howard, P., & Charlesworth, S. (2022). PCF 8: Palliative care formulary.
  3. Davis, M. P., et al. (2024). Opioid analgesic dose conversion ratios. Support Care Cancer, 32, 542.
  4. Adams, M. C. B., et al. (2025). NIH HEAL morphine milligram equivalent calculator. PAIN, 166(8), 1729-1737.
  5. Faculty of Pain Medicine. (n.d.). Opioid rotation guidelines.
  6. Bhatnagar, M., & Pruskowski, J. (2024). Opioid equivalency. In StatPearls.
  7. MD Anderson Cancer Center. (n.d). Peri-operative pain management.
  8. Australian Journal of General Practice. (2024). Morphine toxicity and opioid rotation.
  9. Doulton, B. (2014). Pharmacologic management of breakthrough cancer pain. Can Fam Physician, 60(12), 1111-1114.
  10. Fraser Health Authority. (n.d.). Principles of opioid management.
  11. Caraceni, A., et al. (2023). Parenteral opioids in cancer pain management.
  12. Webber, K., et al. (2014). Breakthrough pain assessment. J Pain Symptom Manage.
  13. Chou, R., et al. (2023). ASCO opioid guidelines for cancer pain. J Clin Oncol.
  14. Mercadante, S., et al. (2024). Methadone rotation for refractory pain.
  15. Prommer, E. (2024). Methadone vs other opioids for bone pain.
  16. Saunders, C. (2010). Total pain management.
  17. IASP Task Force. (n.d.). Multidisciplinary pain management.
  18. Twycross, R., & Wilcock, A. (2014). Alternative routes to oral opioid administration in palliative care. Pain Medicine, 15(7), 1129-1153.
  19. Caraceni, A., et al. (2023). The use of parenteral opioids in cancer pain management. PMC.
  20. NHS England North West. (2020). Palliative care pain & symptom control guidelines.
  21. Twycross, R., et al. (2022). Palliative Care Formulary, 7th ed. (CSCI to fentanyl patch 6-12hr overlap).
  22. NHS Specialist Pharmacy Service. (2021). Fentanyl patch conversion guidelines (12-24hr peak).
  23. Wilcock, A., et al. (2022). Morphine slow release onset 3-4hrs. PCF 8.
  24. Faculty of Pain Medicine, ANZCA. (2021). Opioid patch overlap protocols.
  25. Correll, D. J., et al. (2004). Burst ketamine to reverse opioid tolerance in cancer pain. Journal of Pain and Symptom Management, 27(2), 176-178.
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