Opioid dose conversions from parenteral to oral or transdermal routes

The task

Converting an opioid dose from parenteral (SC/IV) to oral (or vice versa) requires accounting for the reduced oral bioavailability of most opioids compared with the parenteral route - a distinct calculation from converting between different opioid drugs at the same route
A common, high-stakes calculation in palliative care - required whenever a patient transitions between routes (e.g. oral to subcutaneous as swallowing becomes difficult near end of life, or subcutaneous back to oral if the patient stabilises)

Oral opioids undergo first-pass hepatic metabolism, reducing the proportion of the dose reaching systemic circulation compared with the parenteral route, which bypasses this - the oral dose required for an equivalent effect is therefore higher than the parenteral dose

Oral to subcutaneous/IV morphine: approximately 2-3:1 (divide the oral dose by 2-3 for the parenteral equivalent) - the exact ratio, and ratios for other opioids, should always be checked against a current reference (e.g. Therapeutic Guidelines: Palliative Care) rather than relied upon from memory alone given the clinical stakes of an error
When converting and switching opioid simultaneously (e.g. oral morphine to subcutaneous oxycodone), apply both conversions in sequence, and still reduce the final calculated dose to account for incomplete cross-tolerance

Always reduce the calculated equivalent dose (typically by 25-50%) and titrate to effect when converting route and/or drug, rather than converting to the exact calculated equivalent - overestimating the required dose risks life-threatening toxicity
Document the conversion calculation clearly, and involve a second clinician/pharmacist check for complex conversions, given the recognised risk of calculation error in this high-stakes setting

Opioid toxicity, breakthrough dose calculation (typically 1/6-1/10 of the total 24-hour dose regardless of route), syringe driver/subcutaneous infusion setup

Opioid conversion errors are a recognised, preventable source of serious harm in palliative care - the calculation should be treated with the same rigour as any other high-risk medication calculation, ideally with an independent check rather than relying on a single clinician's arithmetic alone

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