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Fentanyl patches (Duragesic) will be available on the PBS from this August. They are on restricted benefit for chronic severe disabling pain, which is associated with proven malignant neoplasia that is unresponsive to non-narcotic analgesics in a patient requiring parenteral opiod treatment. Their use will be mainly restricted to oncology/palliative care patients and not for non malignant pain.
Fentanyl is a synthetic opioid and is about 30 times more potent than parenteral morphine. Once the patch is applied to the skin the drug diffuses into the subcutaneous tissues and forms a reservoir. It is then absorbed into the systemic circulation and takes 17-24 hours to reach optimal plasma levels. The patch then provides a constant analgesic dose for a further 48 hours. It is worth remembering that a Duragesic patch has a half-life of about 17 hours after it is removed. This is due to the formation of the subcutaneous reservoir in the skin. Thus Fentanyl patches are the Queen Mary of the analgesic world, ie. very slow to get up to speed and equally slow to stop.
Conversions
The patch comes in four sizes - 25mcg/hr, 50, 75 and 100. When transferring a patient from morphine to Fentanyl there is a conversion table of equivalent doses shown below.
A quick approximate method of conversion is to divide the total 24 hour dose of morphine by 3.6, e.g 360mg oral morphine divided by 3.6 equals 100mcg/hr patch. It should be noted that there is wide variation in morphine to Fentanyl conversion, therefore it is safer to underestimate the size of patch required.
Benefits
The benefits of Fentanyl patches over slow-release morphine tablets are:
- Less constipation (statistically significant)
- Less nausea (anecdotal evidence)
- Less sedation (anecdotal evidence)
- Patient preference.
Useful tips
- In opiod-naive patients it is good practice to adjust the dose of analgesics required with four hourly morphine first to find the 24 hour morphine requirement and then convert to the equivalent Fentanyl patch.
- If patients require less than 40mg oral morphine per day then even a 25mcg/hr patch may be 'too strong' and cause side effects.
- Fentanyl patches are very unhelpful with unstable pain. Again it is good practice to use four hourly morphine in this situation until analgesic requirements are steady and then convert morphine to the equivalent patch.
- When changing from morphine to Fentanyl the patients can experience opioid withdrawal, eg. abdominal cramps, sweats and flu-like symptoms. This is abated with a dose of four hourly morphine and should only last a few days.
- A small number of patients find the patches last 48 hours and not 72 hours. In these instances it is appropriate to replace the patch every 48 hours rather than increasing the patch size.
- Due to the heat in summer the patches can slide off. Do not use Opsite over the patch to keep it in place, this will cause a faster delivery of Fentanyl and possible overdose. Instead stick micropore tape around the outside of the patch and not directly over the drug reservoir. Incidentally, direct heat, such as a hot water bottle can also increase absorption.
- When changing from morphine, put a patch on at night with the usual dose of bedtime morphine then use oral morphine as needed for breakthrough pain.
- Cut hair rather than shave the skin before applying a patch as shaving will disrupt the top fatty layer of skin and therefore increase the rate of absorption.
- It is possible to increase the dose above 100mcg/hr by using more than one patch at a time.
Relative costs
Duragesic is cheaper than equivalent slow-release morphine for patches of 50mcg/hr and above.
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See also Fentanyl Patches
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