Tolerance is defined as the need for increasing doses of an opioid to achieve the same effect. It is a normal physiological phenomenon, occurring as a consequence of the opioid signally system’s ability to adapt. Tolerance to side effects such as respiratory depression and nausea is beneficial to patients. Tolerance to analgesia is not desirable.
Why is tolerance important?
As we discovered in Week 1, tolerance occurs at different rates according to the effect and the opioid concerned. Tolerance to respiratory depression occurs within days whilst tolerance to analgesia tends to occur with prolonged use. Traditionally, therefore, it was not considered a significant issue in patients with a terminal diagnosis. As cancer treatments evolve and life expectancies increase accordingly, opioid tolerance in cancer pain management has gained more recognition.
Continuing to escalate the dose may have a temporary analgesic effect but this is outweighed by the risk of side effects. Doses may be increased too rapidly for tolerance to respiratory depression to develop. Opioid induced hyperalgesia may also occur and the fact that pain relief is short-lived is distressing for patients.
Managing opioid tolerance
When tolerance to opioids has developed, these drugs are no longer effectively managing the pain. Managing opioid tolerance is challenging and specialist advice should be taken. It is important that clinicians recognise (or at the least suspect) tolerance and seek further advice.
Careful dose reduction should be considered but if pain remains uncontrolled an opioid switch or conversion to an NMDA antagonist such as methadone may be appropriate. Inpatient monitoring and psychological support may be required.
Safe opioid reduction
The safest way to reduce an opioid is to use a 12 hourly MR preparation and gradually reduce the dose, every 3 days by 30-50%. This can be done more quickly for inpatients, under close supervision. Patients who are frail, have less frequent review or who have been on opioids at high doses for a long time will require a slower dose reduction plan.
Try the quiz in the next step to see how you are getting on when opioids are not working.
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