Common Registration Assessment, Part 1

Morphine sulfate oral solution volumes and 4-hourly doses for the GPhC assessment

Three original practice questions, with the working shown. Read the stated prescription or plan before calculating.

Published 22 October 2026. Independent revision material, not GPhC questions, not for patient care.

Close-up of a white pill bottle without a label and capsules lying next to it
Photo from Pexels, used under the Pexels licence. It is a stock image and does not show the exercise.

The BNF lists a morphine sulfate oral solution at 10 mg in 5 mL.

The BNF gives the usual adult starting dose of an immediate-release oral morphine as 10 mg every 4 hours, adjusted to response, with a reduced 5 mg every 4 hours used for frail patients.

Each question fixes the values needed for its arithmetic. Write the unit beside every step and keep a single dose separate from a daily total. All prescriptions and patients here are fictional; these are revision materials, not for patient care.

The method

  1. Identify the exact concentration, dose and frequency supplied in the question.
  2. Convert the dose in mg to a volume: at 10 mg in 5 mL, each 1 mg is 0.5 mL.
  3. Apply the 4-hourly frequency: six doses spread across 24 hours.
  4. Scale to the stated duration only when a supply is requested.
  5. Check the volume is measurable and the arithmetic balances before reporting it.

Three practice questions

Work each one on paper first, then open the answer. All patients and prescriptions are fictional.

Question 1

A fictional prescription specifies morphine sulfate 10 mg every 4 hours using oral solution 10 mg in 5 mL. What volume is one dose?

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Answer: 5 mL

Working: 10 mg ÷ 10 mg in 5 mL: 10 × (5 ÷ 10) = 5 mL per dose.

Question 2

A fictional prescription specifies morphine sulfate 10 mg every 4 hours for 24 hours. How many doses is that in a day, and what volume of the 10 mg in 5 mL solution is used in total?

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Answer: 6 doses, 30 mL in 24 hours

Working: 24 ÷ 4 = 6 doses. Each dose is 5 mL. 6 × 5 = 30 mL.

Question 3

A fictional plan gives a frail patient morphine sulfate 5 mg every 4 hours using oral solution 10 mg in 5 mL for 3 days. What total volume is the course?

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Answer: 45 mL

Working: 5 mg is 5 × 0.5 = 2.5 mL per dose. 24 ÷ 4 = 6 doses a day, so 6 × 2.5 = 15 mL a day. 15 × 3 = 45 mL.

Exam approach

Liquid doses are two conversions in one: mg to volume, then volume to the day. Keep the concentration in view at both steps.

Where marks are lost

  • Reading 10 mg in 5 mL as 10 mg per 1 mL.
  • Counting four doses a day instead of six for an every-4-hours plan.
  • Reporting a daily volume when the question asks for the whole course.

Frequently asked questions

Are these official assessment questions?

No. They are original revision exercises, not official GPhC questions and not for patient care.

Do I need to memorise the concentration?

The exercise supplies it. In the assessment, read any given product information exactly as written.

How should I check the answer?

Check the requested unit, then work backwards from the volume to the stated dose and frequency.

Sources

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

See also the formula sheet, the eight sample questions and the approved calculator page.