Opioid conversion questions appear in virtually every SCE Palliative Medicine sitting. The ratios must be memorised — there is no time during the exam to derive them from first principles, and the Palliative Care Formulary will not be available to you.
The core conversion ratios
All conversions reference oral morphine equivalence as the standard unit.
Oral morphine to oral oxycodone: the ratio is 1.5 to 1. That is, 30 mg of oral morphine is equivalent to 20 mg of oral oxycodone. Oxycodone is one and a half times more potent than morphine by the oral route.
Oral morphine to subcutaneous morphine: the ratio is 2 to 1. That is, 30 mg of oral morphine is equivalent to 15 mg of subcutaneous morphine. The increased bioavailability of the subcutaneous route means half the dose is required.
Oral morphine to subcutaneous oxycodone: the ratio is 1.5 to 1 (oral to oral) then 2 to 1 (oral to SC). In practice, oral morphine 30 mg is equivalent to subcutaneous oxycodone 10 mg. Some clinicians use a direct 3 to 1 ratio (oral morphine to SC oxycodone).
Oral morphine to subcutaneous diamorphine: the ratio is 3 to 1. That is, 30 mg of oral morphine is equivalent to 10 mg of subcutaneous diamorphine. Diamorphine's high solubility makes it preferred for syringe driver use in the UK where available.
Oral morphine to transdermal fentanyl: the standard conversion uses a table. As a general rule, oral morphine 60 mg per 24 hours is approximately equivalent to a fentanyl 25 microgram per hour patch. The patches deliver a continuous dose over 72 hours. When converting, calculate the total 24-hour oral morphine equivalent and divide by 2.4 to get the approximate patch dose in micrograms per hour. However, the PCF conversion table should be used in clinical practice — the exam tests the standard published conversions.
Oral morphine to subcutaneous alfentanil: the ratio is 30 to 1. That is, 30 mg of oral morphine per 24 hours is equivalent to 1 mg of subcutaneous alfentanil per 24 hours. Alfentanil is used in syringe drivers for patients with renal impairment because it has no active metabolites.
Oral morphine to oral methadone: this is the most complex conversion because the ratio is non-linear — it increases at higher morphine doses. For oral morphine up to 90 mg per day, the ratio is approximately 4 to 1. For 90 to 300 mg per day, the ratio increases to approximately 8 to 1. Above 300 mg per day, the ratio may be 12 to 1 or higher. Methadone conversions should always be performed under specialist supervision. The exam tests your knowledge of the non-linear ratio and the need for specialist involvement.
Syringe driver calculations
A common exam question provides a patient on oral opioids who can no longer swallow, and asks you to prescribe a 24-hour syringe driver. The steps are: calculate the total 24-hour oral morphine equivalent, convert to the subcutaneous equivalent using the appropriate ratio, prescribe this as a continuous subcutaneous infusion over 24 hours, and prescribe a breakthrough dose (one-sixth of the total 24-hour dose as an as-needed subcutaneous injection).
For example: a patient on oral morphine modified-release 60 mg twice daily (total 120 mg oral morphine per 24 hours) who needs conversion to a syringe driver. Subcutaneous morphine equivalent is 120 divided by 2 equals 60 mg over 24 hours. Breakthrough dose is 60 divided by 6 equals 10 mg subcutaneous morphine as needed.
Drug compatibility in syringe drivers
The exam also tests which drugs can be mixed in the same syringe driver. Common palliative care combinations include morphine or diamorphine with cyclizine, haloperidol, levomepromazine, midazolam, or hyoscine butylbromide. Known incompatibilities include cyclizine with high concentrations of diamorphine (precipitation risk) and dexamethasone with most other drugs (must be given separately or via a second syringe driver).
iatroX's SCE Palliative Medicine bank includes dedicated opioid conversion and syringe driver calculation questions. The adaptive algorithm ensures these high-yield topics are weighted appropriately. All included at £29 per month or £99 per year.