5th November 2024

Use of opioids

Two key principles in opioid management of pain are:

1. Consideration of the 24-hour total dose and calculation of PRN doses on the basis of this
2. Conversion from one opioid to another, maintaining equivalent doses

Opioid dose calculations

When working with doses, the first step is to calculate total dose received over 24 h = total regular + total PRN

Opioid conversions

The following suggested conversion rates between opioids are taken from the Scottish Palliative Care Guidelines. Note that the BNF suggests a dose reduction of 25-50% when switching between opioids if there is a concern of opioid toxicity. The Scottish Palliative Care Guidelines suggest a dose reduction of up to 30% with re-titration when there is a risk of toxicity, in the frail or elderly, or just because the pharmacokinetics are different between drugs.

Patches can be difficult to remember, and the above equivalences are approximations. Here are two possible approaches:

1. Remember how much is equivalent to the smallest patch and multiple up for bigger patches.
   For example:
   Remember, a buprenorphine 5 mcg patch is approximately equivalent to 12 mg morphine in 24 h
   Work out how much a buprenorphine 15 mcg patch is equivalent to by multiplying by 3
   3 x 12 mg = 36 mg morphine in 2 4h
2. Ensure the time frame and units are consistent, i.e. mg over 24 hours, then use the conversion
   For example: buprenorphine 5 mcg/hr patch
   Total dose in 24 h = 5 mcg x 24 = 120 mcg = 0.12 mg
   Conversion factor to PO morphine (in 24h) = approx. x100
   0.12 mg x 100 = 12 mg/24h PO morphine

Pain management in renal impairment

Opioid accumulation is a potential reversible cause of deterioration (for example becoming drowsy or confused) in palliative patients. Morphine’s active metabolite accumulates in renal failure, and therefore renal function must be taken into account when prescribing. Different opioids are more or less appropriate for patients with renal failure, and when making dose adjustments a balance must be struck between adequate pain control and opioid toxicity.