Conversion ratios: Why is it so challenging to construct opioid conversion tables?

Standardizing opioid management is challenging due to the absence of a ceiling dose, the unknown ideal therapeutic plasma level, and the lack of an clear relationship between dose and therapeutic response. Opioid rotation or conversion, which is switching from one opioid, route of administration, or both, to another, to improve therapeutic response and reduce toxicities, occurs in 20-40 percent of patients treated with opioids. Opioid conversion is often needed when there are adverse effects, toxicities, or inability to tolerate a certain opioid formulation. A majority of patients benefit from opioid conversion, leading to improved analgesia and less adverse effects. There are different published ways of converting opioids in the literature. This review of 20 years of literature is centered on opioid conversions and aims to discuss the complexity of converting opioids. We discuss study designs, outcomes and measures, pain phenotypes, patient characteristics, comparisons of equivalent doses between opioids, reconciling conversion ratios between opioids, routes, directional differences, half-lives and metabolites, interindividual variability, and comparison to package insert information. Palliative care specialists have not yet come to a consensus on the ideal opioid equianalgesic table; however, we discuss a recently updated table, based on retrospective evidence, that may serve as a gold standard for practical use in the palliative care population. More robust, well-designed studies are needed to validate and guide future opioid conversion data.