Common Registration Assessment, Part 1

Clonidine tablet and liquid strengths and daily doses for the GPhC assessment

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

Published 28 October 2026. Independent revision material, not GPhC questions, not for patient care.

Green pill bottle with white tablets scattered on a blue surface
Photo from Pexels, used under the Pexels licence. It is a stock image and does not show the exercise.

The NHS lists clonidine tablets in 25 microgram and 100 microgram strengths, and a liquid containing 50 micrograms in 5 mL; 100 microgram tablets can be halved for a 50 microgram dose.

The NHS lists the usual adult dose for migraine prevention or menopausal hot flushes as 50 to 75 micrograms twice a day, and for high blood pressure a starting dose of 50 to 100 micrograms 3 times a day, up to a maximum of 1.2 mg in total per day.

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 prescription specifies clonidine 50 micrograms twice a day, using half of a 100 microgram tablet per dose, for 28 days. How many whole 100 microgram tablets are used?

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Answer: 28 tablets

Working: Half a tablet × 2 doses = 1 tablet per day; 1 × 28 days = 28 tablets.

Question 2

A fictional plan is clonidine 75 micrograms twice a day for 14 days. What is the total amount taken, in micrograms and in mg?

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Answer: 2,100 micrograms (2.1 mg)

Working: 75 × 2 = 150 micrograms per day; 150 × 14 = 2,100 micrograms; 2,100 ÷ 1,000 = 2.1 mg.

Question 3

A fictional prescription uses clonidine liquid 50 micrograms in 5 mL, at a dose of 100 micrograms three times a day. How many mL are needed per day?

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Answer: 30 mL

Working: 100 micrograms = 10 mL per dose (50 micrograms per 5 mL × 2); 10 × 3 doses = 30 mL.

NHS: How and when to take clonidine

https://www.nhs.uk/medicines/clonidine/how-and-when-to-take-clonidine/

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

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More calculation topics

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