Common Registration Assessment, Part 1
Senna tablet and syrup quantity 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 describes ordinary senna tablets containing 7.5 mg and syrup containing 7.5 mg in 5 mL. Product strengths can differ, so use the label supplied in the question.
The NHS notes that people aged 12 to 17 need a doctor or pharmacist recommendation for bought senna. These are fictional adult regimens, not advice to increase the dose or continue treatment.
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 adult prescription states senna 15 mg at bedtime for 7 nights, using 7.5 mg tablets. What quantity is needed?
Show the answer
Answer: 14 tablets
Working: 15 ÷ 7.5 = 2 tablets nightly; 2 × 7 = 14 tablets.
Question 2
A fictional adult prescription states 7.5 mg nightly for 5 nights, using 7.5 mg/5 mL syrup. What volume is required?
Show the answer
Answer: 25 mL
Working: 7.5 mg corresponds to 5 mL nightly; 5 × 5 = 25 mL.
Question 3
A fictional adult prescription states 15 mg nightly for 3 nights, using 7.5 mg/5 mL syrup. What total volume is needed?
Show the answer
Answer: 30 mL
Working: 7.5 ÷ 5 = 1.5 mg/mL; 15 ÷ 1.5 = 10 mL nightly; 10 × 3 = 30 mL.
NHS: How and when to take senna
https://www.nhs.uk/medicines/senna/how-and-when-to-take-senna/
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.
More calculation topics
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- Dilution calculations with C1V1 = C2V2
- Infusion rate calculations in mL per hour
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See also the formula sheet, the eight sample questions and the approved calculator page.