Opioid Equianalgesic Conversion: Why It Isn't a Calculator
A reference explanation of opioid equianalgesic conversion — how morphine milligram equivalents work, why published conversion tables disagree, and why incomplete cross-tolerance makes automated conversion unsafe.
This page explains why opioid equianalgesic conversion is not something a public calculator should do. It produces no dose or equivalence figure. Opioid rotation must be performed by a prescriber or clinical pharmacist who can apply the reduction for incomplete cross-tolerance and account for the individual patient. If you are managing pain and your current regimen is not working, speak to your prescriber rather than adjusting doses yourself.
Formula & Reference
| Variable | Symbol | Formula | Units |
|---|---|---|---|
| Opioid Equianalgesic Conversion: Why It Isn't a Calculator | — | Concept reference — no conversion output | concept |
Step-by-Step Examples
They express different opioids in a common unit, usually oral morphine milligram equivalents.
- The intent is to allow rotation between opioids without losing analgesia
- Each drug is assigned a ratio relative to oral morphine
- In principle, a patient's total daily dose can be restated in the new drug
- In practice, the ratios themselves are the problem
Different authoritative sources give materially different ratios for the same drug.
- Conversion ratios were derived from small single-dose studies in varied populations
- Ratios differ between acute and chronic dosing
- Methadone is the extreme case — its ratio changes with the dose being converted from, and it accumulates unpredictably
- Transdermal fentanyl conversions vary between manufacturers and guidelines
- Two clinicians using two reputable references can arrive at doses differing by a factor of two or more
This is the reason automated conversion kills people.
- Tolerance to one opioid does not transfer fully to another
- Standard practice is to reduce the calculated equianalgesic dose by 25–50%
- For methadone the reduction is larger and the titration far slower
- A calculator that outputs the raw equianalgesic figure omits the single most important safety step
- Deaths have occurred from opioid rotation performed on calculated equivalence alone
Real-World Applications
Common Mistakes to Avoid
The reduction for incomplete cross-tolerance is not optional refinement — it is the difference between a safe rotation and a fatal one.
Methadone's conversion ratio is non-linear, rising as the previous opioid dose rises, and its long and variable half-life causes delayed accumulation. It requires specialist management.
Renal and hepatic function, age, respiratory status, concurrent sedatives, genetic variation in metabolism, and the reason for rotation all change the appropriate dose.