Sometimes it is necessary to switch from one opioid to another. This step explains the principles underpinning opioid switching.
Morphine is the first-line opioid of choice but sometimes it is necessary to switch opioids, for instance if the patient has become opioid toxic or if they want the convenience of a transdermal formulation.
Why switch opioids
Switching opioids is not an effective way of managing most side effects. The exception is opioid toxicity due to the accumulation of metabolites. In this situation, conversion to an opioid with less toxic metabolites is justified. Sometimes opioids are switched to improve pain control, but this should only be done under specialist guidance.
Opioid conversion ratios
To safely switch opioids, professionals must be aware of conversion ratios and be able to access specific opioid conversion guidance.
Opioids differ in their potency. For instance, according to the Palliative Care Formulary, 10mg of codeine is equivalent to 1mg of morphine and 1mg of oxycodone is equivalent to 1.5mg of morphine. These ratios are for guidance only. The response to opioids varies between individuals due to genetic differences in opioid receptor expression and co-morbidities such as renal failure. For this reason a patient MUST be carefully monitored and reviewed after the opioid is switched, just as they would when opioids are initiated or titrated.
Some guidelines recommend the dose is reduced at the time of switching to reduce the risk of opioid toxicity. This decision depends on how well the pain is controlled and at the time. If a patient is opioid toxic a dose reduction should be considered, guided by the patient’s pain. As a minimum it is good practice to round down to the most practical dose and ensure an immediate-release (prn) opioid is also prescribed.
In practice this means that when converting from morphine to oxycodone, the dose is often halved rather than reducing by the 1.5 conversion factor referred to above. This also makes for an easier calculation and therefore reduces the risk of a prescribing error. Using published conversion charts and tables also reduce the risk of error.
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