![]() There is no evidence-based recommendation for an appropriate reduction. Incomplete cross-tolerance can occur due to variability in opioid binding. When switching between opioids, equianalgesic conversions may overestimate the potency of the new opioid due to incomplete cross-tolerance. Opioid metabolism and excretion do differ among the opioids therefore, alterations in drug disposition will alter the relative potencies of different opioids. Patient-specific factors: No equianalgesic table is able to take into account patient-specific factors - primarily hepatic function, renal function, and age.10 These discrepancies are a factor of both references using old data (single-dose studies) and an overall paucity of data in chronic dosing studies. Equianalgesic Discrepancies: There are significant discrepancies in equianalgesic dosing tables, with even FDA-approved drug labels not demonstrating agreement.9 In patients with very high opioid requirements, the difference between 25% and 50% can be a very significant discrepancy. Cross-tolerance: Many references recommend a cross-tolerance reduction between 25-50% when converting between unlike opioids.In the case of converting morphine to methadone, methadone has a relative potency of 4:1 at lower morphine doses, but becomes much more potent (12:1) in patients converting from very high morphine doses. Dose-dependent conversions: The conversion ratio of certain opioids can be dependent on the dose of the original opioid.These bidirectional differences are not captured in a traditional equianalgesic table. Bidirectional conversions: When converting between certain opioids, the direction of conversion (eg, morphine to hydromorphone versus hydromorphone to morphine) will produce a different conversion ratio.8 Due to drug accumulation, half-life, tolerance, and active metabolites, subsequent chronic administration studies often vary greatly from the original single-dose data. Single-dose studies: Early studies determining equianalgesia were based on single doses, not chronic administation.While these equianalgesic tables are current the "best" solution, their limitations should be emphasized: Reasonable clinical judgment, breakthrough (rescue) opioid regimens, and dose titration are of paramount importanceĪs a clinician, it is important to note that there are significant limitations to equianalgesic conversions and tables. ![]()
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