Common Registration Assessment, Part 1
Trimethoprim tablets, liquid and course lengths 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 lists trimethoprim tablets of 100 mg and 200 mg and a liquid containing 50 mg in 5 mL. The usual adult dose for a urinary tract infection is 200 mg twice a day, and courses range from 3 days for a straightforward infection to 7 to 14 days for severe or complicated ones.
For prevention the usual adult dose is 100 mg once a day at night, and for acne 300 mg twice a day. The indication in the supplied plan decides which dose arithmetic applies.
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 prescription specifies trimethoprim 200 mg twice a day for 3 days, using 200 mg tablets. How many tablets are needed?
Show the answer
Answer: 6 tablets
Working: 200 ÷ 200 = 1 tablet per dose. 1 × 2 × 3 = 6 tablets.
Question 2
A fictional dose of trimethoprim is 200 mg, using the liquid containing 50 mg in 5 mL. What volume is one dose?
Show the answer
Answer: 20 mL
Working: 200 ÷ 50 × 5 = 20 mL.
Question 3
A fictional adult prescription for acne specifies trimethoprim 300 mg twice a day, using 100 mg tablets. How many tablets is one dose, and how many in one day?
Show the answer
Answer: 3 tablets per dose and 6 tablets per day
Working: 300 ÷ 100 = 3 tablets per dose. 3 × 2 = 6 tablets per day.
NHS: How and when to take trimethoprim
https://www.nhs.uk/medicines/trimethoprim/how-and-when-to-take-trimethoprim/
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.