Common Registration Assessment, Part 1
Breakthrough opioid dose calculations
Three original palliative care questions on oral morphine rescue doses from the 24-hour total, with the working shown.
Published 14 October 2026. Independent revision material, not GPhC questions, not for patient care.
This page is about palliative care prescribing in adults using oral morphine. It is not a general pain-relief rule and does not cover other opioids, renal impairment or children. The BNF guidance on palliative care says the usual rescue dose of oral immediate-release morphine for breakthrough pain is one-tenth to one-sixth of the regular 24-hour dose, repeated every 2 to 4 hours as required. For a subcutaneous infusion, the rescue bolus is one-tenth to one-sixth of the total 24-hour subcutaneous dose.
The calculation starts with the whole 24-hour dose, not a single dose. Questions often give twice-daily modified-release doses, so add them first, then work out the range and, if asked, the volume of liquid.
The method
- Add every regular dose in 24 hours to get the 24-hour total.
- Divide the total by 10 for the lower end and by 6 for the upper end of the range.
- If the dose is a liquid, divide the dose in mg by the strength in mg per mL, or scale from the strength per 5 mL.
- For a syringe driver, use the total subcutaneous dose in 24 hours, not the oral dose.
- State the range in mg and check the units before choosing an answer.
Three practice questions
Work each one on paper first, then open the answer. All patients and prescriptions are fictional.
Question 1
A palliative care patient takes modified-release oral morphine 30 mg twice daily. What is the range for the oral immediate-release morphine rescue dose?
Show the answer
Answer: 6 mg to 10 mg
Working: 24-hour dose: 30 × 2 = 60 mg. One-tenth: 60 ÷ 10 = 6 mg. One-sixth: 60 ÷ 6 = 10 mg.
Question 2
A palliative care patient has a 24-hour oral morphine dose of 90 mg. Immediate-release oral solution is 10 mg in 5 mL. What is the range of rescue doses in mg, and what volume gives the upper end?
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Answer: 9 mg to 15 mg, and 7.5 mL for the upper end
Working: One-tenth: 90 ÷ 10 = 9 mg. One-sixth: 90 ÷ 6 = 15 mg. Strength: 10 mg in 5 mL is 2 mg/mL. 15 ÷ 2 = 7.5 mL.
Question 3
A syringe driver gives 45 mg of morphine subcutaneously over 24 hours. The prescriber chooses a rescue dose of one-sixth of the 24-hour dose. What is one dose, and what is the total in 24 hours if 4 rescue doses are given?
Show the answer
Answer: 7.5 mg per dose, and a total of 75 mg
Working: One-sixth: 45 ÷ 6 = 7.5 mg. Four doses: 7.5 × 4 = 30 mg. Total in 24 hours: 45 + 30 = 75 mg.
Exam approach
Find the 24-hour total first, then divide by 10 and by 6 to get the range.
Where marks are lost
- Using one regular dose instead of the full 24-hour total.
- Using an oral total to work out a subcutaneous rescue dose.
- Mixing up the fraction, for example dividing by 16 instead of 6.
- Reporting mg when the question asks for mL of liquid.
Frequently asked questions
What is the usual rescue dose of morphine for breakthrough pain?
The BNF palliative care guidance gives one-tenth to one-sixth of the regular 24-hour dose, repeated every 2 to 4 hours as required, and notes that up to hourly may be needed.
Why check the total in 24 hours when opioids change?
The BNF notes that when reviewing or changing opioid prescriptions, the total dose of opioid taken in a 24-hour period should be considered. Rescue doses count towards that total.
Sources
- BNF, prescribing in palliative care
- GPhC, Common Registration Assessment specification and permitted items for 2026
The questions above are original and use fictional patients and prescriptions. PreRegExamPrep is not affiliated with or endorsed by the General Pharmaceutical Council.
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More calculation topics
- Dose by weight calculations for the GPhC assessment
- Volume to give: liquid medicine calculations
- Percentage strength calculations: w/v, w/w and ratios
- Dilution calculations with C1V1 = C2V2
- Infusion rate calculations in mL per hour
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- Units and units per mL
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- Diluting creams and ointments by weight
- Scaling a formula to a batch size
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- Tablet fractions and split doses
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- Osmolarity from component concentrations
- Direct proportion and cross-multiplication
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- Working backwards from a stated dose
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- Clearance from half-life and volume of distribution
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See also the formula sheet, the eight sample questions and the approved calculator page.