Common Registration Assessment, Part 1

Azathioprine tablet strengths and weight-based doses for the GPhC assessment

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

Published 1 November 2026. Independent revision material, not GPhC questions, not for patient care.

Gloved healthcare worker holding a blood sample in a test tube
Photo from Pexels, used under the Pexels licence. It is a stock image and does not show the exercise.

The NHS describes azathioprine as a medicine used for autoimmune conditions, and treatment needs regular blood test monitoring.

The manufacturer's UK product information lists 25 mg and 50 mg tablets, with a usual starting dose of 1 mg to 3 mg per kilogram of body weight a day.

Weight-based doses rarely match a single tablet, so a stated daily dose is usually built from a combination of the two strengths.

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. Multiply the stated milligrams per kilogram by the stated body weight first.
  2. Build the daily total from the stated tablet strengths, using the fewest tablets.
  3. Calculate one day first, then scale to the stated number of days.
  4. Count each strength separately when a supply uses more than one strength.
  5. Keep milligrams and tablets separate; convert only if the question asks.

Three practice questions

Work each one on paper first, then open the answer. All patients and prescriptions are fictional.

Question 1

A fictional patient weighs 60 kg and starts azathioprine at 2.5 mg per kilogram a day. Using 50 mg tablets, how many tablets is the daily dose?

Show the answer

Answer: 3 tablets

Working: 60 kg x 2.5 mg/kg = 150 mg a day; 150 mg / 50 mg = 3 tablets.

Question 2

A fictional patient takes azathioprine 150 mg once a day. How many milligrams and how many 50 mg tablets is a 28-day supply?

Show the answer

Answer: 4,200 mg; 84 tablets

Working: 150 mg x 28 days = 4,200 mg; 3 tablets x 28 days = 84 tablets.

Question 3

A fictional patient weighs 75 kg and takes 1 mg per kilogram a day. Which combination of 50 mg and 25 mg tablets makes the daily dose, and how many of each strength is a 28-day supply?

Show the answer

Answer: One 50 mg tablet plus one 25 mg tablet a day; 28 of each strength

Working: 75 kg x 1 mg/kg = 75 mg a day; 50 mg + 25 mg = 75 mg; 1 tablet of each strength x 28 days = 28 of each.

NHS: Azathioprine

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

Where marks are lost

  • Multiplying the dose per kilogram by the wrong weight or skipping the weight step.
  • Rounding a weight-based dose to a tablet count without checking which strengths are stated.
  • 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 weight-based calculation justify changing a real dose?

No. Follow the steps in the exercise. Real doses of this kind are set and monitored by specialists.

How should I check the answer?

Check the requested unit, then work backwards from the quantity to the stated dose, weight 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.

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