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Opiate switching

Evidence

- Up to 80% of patients achieve a successful outcome after switching!


Who to switch?

Shortly after initiation of original opiate where dose is low - trialling a different one may give a different efficacy


Long duration of use leading to less efficacy and less benefit on increasing doses


Tolerance

Remember, tolerance can occur to both the analgesic effects, but also to the central effects which otherwise cause side effects (e.g. constipation may lessen with time)


Incomplete cross tolerance occurs when one opiate works in slightly different ways at different types of mu receptor - facilitating slightly different responses


Equianalgesia

The tables available are based on single dose studies - so applicability can vary

Recommendation is to reduce the dose when changing by 33-50% and titrate to effect

Methadone is far more complicated - particularly due to its long half-life and lack of active metabolites


  1. Calculate the dose for the last 24 hours

  2. Convert this does to the equivalent morphine dose

  3. Then, convert this dose to the new opioid - considering route of administration

  4. Recommend starting converted dose at 50-75% of calculated equianalgesic dose

  5. Prescribe breakthrough as required (1/10th to 1/6th of the total daily dose)

(SEE TABLES AT BOTTOM FOR CONVERSION)


Pearls

Note there is no conversion rate for fentanyl. It is too unpredictable to reliably provide an equianalgesic dose of morphine. Start at 200mcg lozenge and titrate up as required









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