Common Registration Assessment, Part 1
Lactulose dose and bottle supply calculations for the GPhC assessment
Three original practice questions, with the working shown. Read the stated prescription or plan before calculating.
Published 15 October 2026. Independent revision material, not GPhC questions, not for patient care.
NHS guidance gives 15 mL twice daily as the usual adult starting dose for constipation. The dose is already expressed as a volume, so quantity questions do not require a mg-to-mL conversion.
Doses can change with response and indication. Children under 14 should receive lactulose only when their doctor recommends it. Whole-bottle rounding is used only when the question asks for it.
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
An adult prescription states lactulose 15 mL twice daily for 10 days. What total volume is required?
Show the answer
Answer: 300 mL
Working: 15 × 2 = 30 mL daily; 30 × 10 = 300 mL.
Question 2
A fictional adult prescription states 30 mL three times daily for 7 days. If only whole 500 mL bottles may be supplied in this exercise, how many cover the course?
Show the answer
Answer: 2 bottles
Working: 30 × 3 × 7 = 630 mL. 630 ÷ 500 = 1.26; round up to 2 under the stated whole-bottle rule.
Question 3
A doctor-prescribed fictional child regimen states 5 mL twice daily. How many complete days does 200 mL cover?
Show the answer
Answer: 20 days
Working: 5 × 2 = 10 mL daily; 200 ÷ 10 = 20 complete days.
NHS: How and when to take lactulose
https://www.nhs.uk/medicines/lactulose/how-and-when-to-take-lactulose/
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.