Equianalgesic doses of opioids - their use in clinical practice.

Abstract

Opioid rotation is an established means of managing the side-effects of strong opioids.1 From its origins in palliative care, the concept has moved into the fields of acute and chronic pain in cases where it is felt that changing a strong opioid may confer a benefit or if it becomes necessary in the context of co-morbidity. In an acute pain setting, for example, if a patient is undergoing an operation that may result in a reduction in their renal function then it may be necessary to change their opioid to one without metabolites that can cause problems with accumulation. In a pain clinic, it may be that a patient has developed tolerance over time to the analgesic effect of an opioid that was initially helpful, and the clinician believes that opioid rotation rather than dose escalation is an appropriate way to manage this. In a hospice, a patient may be receiving acceptable pain relief using a strong opioid, but may also be struggling with side-effects; therefore, a change to a different opioid, anticipated to have fewer side-effects, is planned. The question, therefore, clinicians often find themselves asking is, ‘If I am starting with “a” opioid at “b” dose, what dose of “x” opioid do I need to prescribe to have the same analgesic effect?’. This information can be derived from the equianalgesic dose, which is the dose of two opioids required to produce the same analgesic effect.2 There are a number of sources of information available to guide clinicians. The British National Formulary (BNF) publishes tables comparing a number of strong opioids with morphine as the ‘gold standard’.3 The Palliative Care Formulary provides more comprehensive information, and for some drugs information is given in the summary of product characteristics.4 In the wider literature there are summary tables,5,6 diagrams7,8 and now online resources including medically orientated smartphone applications. Importantly, however, the BNF and most publications display notices of caution alongside such tables. The BNF notes, ‘these equivalences are intended only as an approximate guide; patients should be carefully monitored after any change in medication and dose titration may be required’. There are a number of reasons why caution is raised alongside the publication of such tables, and these relate to patient factors and issues surrounding the information used to create the tables.