Common Registration Assessment, Part 1
Macrogol sachet schedules and water volumes for the GPhC assessment
Three original practice questions, with the working shown. Read the stated prescription or plan before calculating.
Published 16 October 2026. Independent revision material, not GPhC questions, not for patient care.
The NHS describes full-strength macrogol sachets for constipation as 1 sachet taken 1 to 3 times a day. For faecal impaction the schedule starts at 4 sachets on the first day, rises by 2 sachets a day and stops at a maximum of 8 sachets a day.
Each sachet is dissolved in 125 mL of water, and the bottled liquid is measured as 25 mL diluted in 100 mL of water, taken 1 to 3 times a day. Escalation schedules come from a prescriber, not from arithmetic.
Each question fixes the values needed for its arithmetic. Write the unit beside every step and distinguish a single dose from a daily total or course supply. All prescriptions and patients here are fictional; these are revision materials, not for patient care.
The method
- Identify the exact formulation, strength, dose and frequency supplied in the question.
- Convert an X mg/5 mL strength to mg/mL if a dose-to-volume conversion is needed.
- Calculate one dose first; for tablets or capsules divide prescribed mg by mg per unit.
- Scale to the stated frequency and duration only when a total quantity is requested.
- Apply whole-container or complete-day rounding only if the question specifies it.
Three practice questions
Work each one on paper first, then open the answer. All patients and prescriptions are fictional.
Question 1
A fictional supplied faecal impaction schedule uses full-strength sachets: 4 sachets on day 1, increasing by 2 sachets a day, to a maximum of 8 sachets a day, for 3 days. How many sachets does the schedule use in total?
Show the answer
Answer: 18 sachets
Working: Day 1: 4. Day 2: 4 + 2 = 6. Day 3: 6 + 2 = 8, which is the maximum. 4 + 6 + 8 = 18 sachets.
Question 2
A fictional daily amount is 6 sachets, each dissolved in 125 mL of water. What total volume of water is used in the day?
Show the answer
Answer: 750 mL
Working: 6 × 125 = 750 mL.
Question 3
A fictional prescription specifies macrogol liquid 25 mL, diluted in 100 mL of water, taken 3 times a day. What total volume of the medicine is taken each day, and what total volume of water is used to dilute it?
Show the answer
Answer: 75 mL of medicine and 300 mL of water
Working: 25 × 3 = 75 mL of medicine. 100 × 3 = 300 mL of water.
NHS: How and when to take macrogol
https://www.nhs.uk/medicines/macrogol/how-and-when-to-take-macrogol/
Where marks are lost
- Using a remembered strength or schedule instead of the stated product.
- Reporting a daily total when the question asks for one dose or the course.
- Treating arithmetic as evidence that a real prescription is clinically suitable.
Frequently asked questions
Are these official assessment questions?
No. They are original revision exercises, not official GPhC questions and not for patient care.
Can a quantity calculation justify a formulation switch?
No. Follow the formulation and prescription in the exercise. Real switches and dose changes need clinical checks.
How should I check the answer?
Check the requested unit, then work backwards from the quantity to the stated dose and duration.
Sources
The questions above are original and use fictional patients and prescriptions. PreRegExamPrep is not affiliated with or endorsed by the General Pharmaceutical Council.
Practise until the method is automatic
Try 15 free questions with worked answers. No sign-up required.
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See also the formula sheet, the eight sample questions and the approved calculator page.