Tolerance to different effects of a medication can develop at different rates in a single individual.

• Tolerance to the analgesic effects of morphine occurs slowly.
• Tolerance to drowsiness develops within two or three days.
• Tolerance to opioid induced constipation does not develop at all.

This is also true for different opioids, particularly of different classes.

Tolerance to analgesia may occur more slowly than tolerance to adverse effects.  This means changing to a different major opioid (‘rotation’ or ‘substitution’) can provide relief from some adverse effects (such as neuroexcitability) by allowing a reduction to OME opioid dose with no loss of analgesia.  This describes incomplete cross tolerance.

In order to account for incomplete cross tolerance, once the equivalent dose of the new drug has been calculated this dose should be decreased by 25%.

Please contact the specialist paediatric palliative care team for advice on opioid rotation.