Opioid Cross-Tolerance

* While pharmacologic tolerance may develop to the opioid in use, tolerance may not be as marked relative to other opioids

* Incomplete cross-tolerance is likely due to subtle differences in:

o The molecular structure of each opioid
o The way each interacts with the patient’s opioid receptors

* Consequently, when switching opioids, there may be differences between published equianalgesic doses of different opioids and the effective ratio for a given patient

* Start with 50% to 75% of the published equianalgesic dose of the new opioid to compensate for incomplete cross-tolerance and individual variation, particularly if the patient has controlled pain

o If the patient has moderate to severe pain, do not reduce the dose as much
o If the patient has had adverse effects, reduce the dose more

* An important exception is methadone, which appears to have higher than expected potency during chronic dosing (compared with published equianalgesic doses for acute dosing)

o Start with 10% to 25% of the published equianalgesic dose
o Titrate appropriately to achieve pain control