Common Registration Assessment, Part 1

Carbamazepine liquid volumes and daily doses for the GPhC assessment

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

Published 24 October 2026. Independent revision material, not GPhC questions, not for patient care.

A single pill close up
Photo from Pexels, used under the Pexels licence. It is a stock image and does not show the exercise.

The BNF starting dose of carbamazepine for epilepsy is 100 to 200 mg once or twice a day, rising in steps of 100 to 200 mg every 2 weeks to a usual 0.8 to 1.2 g daily in divided doses.

For short periods when oral dosing is not possible, the BNF allows rectal use up to 1 g daily in four divided doses for up to 7 days.

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

  1. Identify the exact formulation, strength, dose and frequency supplied in the question.
  2. Convert an X mg/5 mL strength to mg/mL if a dose-to-volume conversion is needed.
  3. Calculate one dose first; for tablets or capsules divide prescribed mg by mg per unit.
  4. Scale to the stated frequency and duration only when a total quantity is requested.
  5. 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 dose is carbamazepine 400 mg, using the liquid containing 100 mg in 5 mL. What volume is one dose?

Show the answer

Answer: 20 mL

Working: 400 ÷ 100 × 5 = 20 mL.

Question 2

A fictional daily amount is carbamazepine 800 mg in divided doses, using 200 mg tablets. How many tablets is that day?

Show the answer

Answer: 4 tablets

Working: 800 ÷ 200 = 4 tablets.

Question 3

A fictional prescription specifies carbamazepine 200 mg twice daily for 14 days, using 200 mg tablets. How many tablets cover the course?

Show the answer

Answer: 28 tablets

Working: 2 tablets per day × 14 days = 28 tablets.

NHS: Carbamazepine

https://www.nhs.uk/medicines/carbamazepine/

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.

Try 15 free questionsSee pricing

More calculation topics

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