8th January 2024

Routes and dose conversion

As mentioned before, there are a number of strong opioids available. Not all drugs are equal. Although some drugs are ‘stronger’ than others, dose titration is a more appropriate method to achieve pain control than switching opioids initially. Opioid rotation is an option where pain is continually not controlled with one drug despite uptitration but would be guided by a specialist. Switching between two different drugs happens frequently in palliative care due to changes in a patient’s condition, such as the development of renal failure. Where patients are already established on analgesia, converting the dose accurately avoids the development of side effects and loss of pain control.

Source: https://gmmmg.nhs.uk/guidance/clinical-guidance-and-pathways/

Converting from one drug to another is possible using the table above for reference. The key steps are:

• Calculating the total opioid dose (maintenance and breakthrough)
• Converting to oral morphine
• Convert to final opioid

Most conversion tables available for various opioids will quote the conversion to or from oral morphine. It is, therefore, safer, simpler, and more accurate to convert between the drug in question and oral morphine before converting to the opioid of choice.

For instance, if a patient takes 240mg of codeine a day, and the equivalent dose of oxycodone was required:

• 240mg ÷ 10 = 24mg equivalent dose of morphine
• 24mg ÷ 2 = 12mg equivalent dose of oxycodone

Even within the same drug, the relative strength varies across different routes. This has implications in clinical practice. Oral is the preferred route for analgesia, but where this is not available (nausea and vomiting, tablet burden, poor absorption, reduced consciousness), alternatives need to be considered, and conversion may be required.

Transdermal route

The transdermal route is an option where the oral route is not available. This involves the application of patches, most commonly fentanyl or buprenorphine. These patches remain in place for days at a time, the duration different between different brands. The absorption of patches can be increased by heat, for instance, in pyrexia, and can cause side effects and toxicity.

Patches reach a steady state over a period of days. They are typically used in stable, well-controlled pain and are not appropriate for unstable pain. However, if pain becomes unstable, abruptly stopping the patch would not be appropriate. Other opioids can be added alongside the patch in that case.

Both fentanyl and buprenorphine patches are only available in certain doses with less flexibility than oral alternatives. Conversion is not as straightforward as a simple equation. This again demonstrates the utility of morphine as a ‘currency’ for opioid conversions because of the widespread availability of conversion information. There may not be an exact equivalent patch compared to the existing dose of opioid, and a slight dose reduction is preferable.

Source: https://gmmmg.nhs.uk/guidance/clinical-guidance-and-pathways/

As you can see, even the lowest-strength fentanyl patch is a comparatively large opioid dose. It is, therefore, recommended not to use fentanyl in opioid naïve patients.

Using the table above to switch from 120mg daily oral oxycodone to a fentanyl patch:

• 120mg ÷ 2 = 60mg equivalent dose of oral morphine
• 60mg = Fentanyl 25mcg/hour patch

When switching between modified release agents and transdermal, the patch should be applied at the same time as the final modified release preparation is taken.

Subcutaneous route

Subcutaneous syringe drivers are widely used in palliative care to deliver a range of medications, not just analgesics. Syringe drivers are dosed over 24 hours – with the prescribed drug being put into an infusion over 24 hours. This provides a constant supply of drugs without peaks and troughs. They are commonly used at the end of life. As required, doses can also be made available subcutaneously. There is no ‘immediate-release’ or ‘modified-release’ version of the injection, and it is the same drug either administered as a bolus or over 24 hours. There are often direct oral to subcutaneous conversion equations available within the same drug (such as oxycodone), so conversion to oral morphine is not always necessary.

Syringe drivers should be commenced immediately if the patient is not receiving opioids or is only taking immediate-release oral opioids. If the patient takes modified-release opioids, the syringe driver should be commenced at the time the next modified-release preparation is due.

Source: https://gmmmg.nhs.uk/guidance/clinical-guidance-and-pathways/