Palliative care: Opioids for pain
Often patients are initially managed on a regular dose of an immediate-release morphine product such as Oramorph solution every four hours. When pain control is stable, they are then switched to a modified-release formulation that is usually given twice daily. The initial dose of modified-release morphine can be calculated by adding up the total amount of oral morphine over 24 hours. This includes regular four hourly doses plus any ‘when required’ doses prescribed for breakthrough pain.
For example, a patient, Paul, has required 10mg oral morphine solution every four hours with two 10mg ‘when required’ doses, for a few days. His 24-hour morphine requirement is 80mg. Therefore his starting dose of modified-release morphine would be 40mg every 12 hours or 80mg every 24 hours.
The BNF advises that the first dose of the modified-release preparation can be given at the same time as the last dose of the immediate-release preparation, or within 4 hours of it. However, in practice there might need to be more of an overlap while the modified-release preparation starts to work. Immediate-release morphine should continue to be prescribed ‘when required’ for breakthrough pain and can be given every 2-4 hours (up to hourly may be needed if pain is severe or in the last days of life). The ‘when required’ dose should be about one-sixth to one-tenth of the total daily dose of modified-release morphine. Additional doses of opioids may also be required when breakthrough pain is predicted such as during a
dressing change.
For our patient, Paul, on modified-release morphine sulphate 40mg every 12 hours, the ‘when required’ dose of immediate-release morphine would be around 10mg every 2-4 hours.
If a patient’s condition declines and they become unable to swallow, a change in route of administration should be considered. Morphine is often the preferred parenteral opioid given continuously using an infusion device called a syringe driver. Diamorphine is also sometimes used; it is more soluble in water compared to morphine and can be dissolved in a smaller volume.
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In practice in the UK, the subcutaneous route is used in preference to the intravenous route. This is in part because inserting an intravenous access device can sometimes be difficult in a palliative context (e.g. if the patient is restless, and/or has poor venous access). In addition, the intravenous route is associated with a broader range of complications compared to the subcutaneous route (e.g. phlebitis, extravasation). Also if access is lost, there may be more of a delay in inserting a new intravenous cannula which is a more specialist skill compared to a subcutaneous access device, which may lead to loss of symptom control. Subcutaneous administration also allows for greater patient mobility, as they are less restricted by the presence of a peripheral intravenous cannula usually in an arm vein.
Fentanyl and buprenorphine are useful alternatives that can be given as transdermal patches if the patient's pain is stable and their dose is unlikely to change rapidly. However serious harm including fatalities have been reported with transdermal opioid preparations due to accidental exposure in the case of fentanyl and medication errors with buprenorphine, and so extra care is required if these are prescribed.
Guidance on conversion between oral morphine and other opioids is given in most palliative care resources. Interpret dose conversion factors with care as they are only approximations, and there is a great deal of inter-patient variability.
Anticipation and management of the side effects of analgesics is an important part of optimising therapy. Nausea and vomiting, sedation and constipation are the most common side effects observed in patients taking opioids. However nausea and vomiting are usually transient and improve after several days. Patients should take long-term laxatives regularly to minimise the risk of constipation.