Skip to 0 minutes and 9 seconds From time to time, we see patients that come to us, and they’re on two different types of opioid. Say, for instance, morphine, modified release for their background pain, and oxycodone– instant release for their breakthrough pain. Now, this isn’t an ideal situation as far as we’re concerned. Can you explain why? I agree, Vicky. I think this is all down to patient safety, really. If you co-prescribe opiates, especially strong opiates, it can cause confusion. People aren’t used to using it. They don’t realise that they’re not equipotent. It can confuse patients, and it can confuse doctors as well, and nurses. There is no doubt that it is not good advice to co-prescribe opiates.
Skip to 0 minutes and 57 seconds And we try, with the best will in the world, to try and just keep it the one opiate for your modified release, and then the breakthrough your pain. There is incidences, of course, when patients can be on fentanyl– Yes. –and that they need morphine. However, that is when you want to be speaking and seeking advice from a specialist. So in summary, we try and teach only one opiate, not to confuse things. And a lot of that is down to patient safety. What would you advise a clinician do if they are considering an opioid switch? That’s a good question. I always tell people to lift the phone and seek some advice.
Skip to 1 minute and 35 seconds There is likely to be local guidelines on what to do with renal impairment, and opiates and renal impairment. And we don’t always use these drugs often, so I think it’s important to get advice. Because sometimes, it’s better to use a well-known opiate and adapt it to the renal function, rather than use one that has less or no metabolites in renal impairment. Because it’s not as safe, because you don’t really– you haven’t used it enough. So I think it’s either be guided with those drugs with the specialist, or try and get some advice on using a well-known drug safely in the renal dysfunction. That’s great advice.
The most common reason to convert opioids is to reduce side effects arising from the accumulation of toxic metabolites. It is therefore an important patient safety skill to master.
In this video Dr Paul Coulter explains why co-prescribing of strong opioids should be avoided and gives advice on switching opioids in renal failure.
Sometimes an opioid is converted to improve medication compliance and patient choice. This was the case when our patient, Terry, started a fentanyl patch. Very occasionally it is done in an attempt to achieve better pain control or reduce side effects by “hitting” different opioid receptors. This should only be done under specialist supervision due to limited evidence and potentially higher costs.
Opioids differ in their analgesic potency but the evidence for precise dose equivalence is low. These studies largely come from the pharmaceutical industry and many have been criticised for their methodological flaws and application to practice. Recent research indicates that some opioid conversions are not bi-directional - that is, the conversion ratio is not the same in both directions. Finally, genetic variation in the expression of opioid receptors means individuals respond differently to opioids.
Opioid conversion tables
Even though opioid rotation is an imprecise art, conversion tables are widely used in practice. Some ratios differ slightly from one table to another, due to the issues described above. Therefore all opioid conversions should be considered approximations and the patient reviewed after 24 hours. To ensure patient safety, we recommend doses be rounded down wherever practically possible.
Morphine, the first-line opioid of choice, it is the benchmark against which potency is compared. Here we consider the most commonly encountered opioids and conversion ratios, comparing oral preparations unless stated otherwise. Conversion to another route of administration (eg oral to subcutaneous) requires a further calculation, as described in week 2.
Codeine/ Dihydrocodeine/ Tramadol to Morphine = 10:1
These weak opioids are approximately 10 times weaker than morphine. In clinical practice this calculation is most frequently undertaken when a patient moves from step 2 to step 3 of the WHO analgesic ladder.
A patient prescribed 2 co-codamol 8mg/500mg tablets four times a day is taking 64mg of codeine in 24 hours, equivalent to 6.4mg of morphine. It is also worth noting that 100mg of tramadol four times a day is equivalent to 40mg of morphine in 24 hours - that is 20mg of modified-release (MR) morphine twice a day.
Morphine to Oxycodone = 1.5:1
This conversion is commonly done in response to renal failure because anecdotal evidence suggests oxycodone is better tolerated than morphine in mild to moderate renal impairment. Recent evidence indicates morphine: oxycodone ratio is 1.5:1. Some conversion tables recommend a 2:1 ratio, which may be safer in terms of calculating the dose.
Morphine to Alfentanil injection = 30:1
Alfentanil injection is thirty times more potent than oral morphine. The conversion is necessary when a patient is in severe renal failure. This conversion is from oral to subcutaneous injection because alfentanil is not available orally.
Morphine to Fentanyl patch = consult guidelines and/ or seek specialist advice
Fentanyl patches should only be prescribed when pain is stable, usually for patient convenience. The lowest strength fentanyl patch is 12 micrograms per hour, which is equivalent to 30mg of oral morphine in 24 hours. Opioid doses less than this are too low for a fentanyl patch. A 25 microgram per hour patch is only appropriate if the patient is taking the equivalent of 60mg of morphine in 24 hours AND the pain is stable.
Note: For other opioid conversions please seek specialist advice.
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