Common Registration Assessment, Part 1

Ethambutol dose by body weight and tablet counts for the GPhC assessment

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

Published 13 November 2026. Independent revision material, not GPhC questions, not for patient care.

A doctor in scrubs talking with a patient in a bright consulting room
Photo from Pexels, used under the Pexels licence. It is a stock image and does not show the exercise.

The manufacturer's UK product information for ethambutol 400 mg tablets states that for primary treatment and prophylaxis, ethambutol is given in a single daily dose of 15 mg per kg of body weight, with other anti-tuberculosis medicines maintained at their recommended doses.

For re-treatment, the same information gives a single daily dose of 25 mg per kg for the first 60 days, reducing to 15 mg per kg from day 61 onwards.

Ethambutol tablets come in 100 mg and 400 mg strengths, so a calculated daily dose is built by combining the two strengths.

Each question fixes the values needed for its arithmetic. Write the unit beside every step and state both the mg total and the tablet split. All prescriptions and patients here are fictional; these are revision materials, not for patient care.

The method

  1. Multiply the stated body weight in kg by 15 mg for primary treatment, or by 25 mg for the first 60 days of re-treatment.
  2. Reduce a re-treatment dose to 15 mg per kg from day 61.
  3. Build the daily dose from 400 mg and 100 mg tablets, using the larger strength first.
  4. Give the answer as a single daily dose, not divided doses.
  5. Remember ethambutol is used with other anti-tuberculosis medicines, not alone.

Three practice questions

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

Question 1

A fictional adult weighs 60 kg and is prescribed ethambutol at 15 mg per kg as a single daily dose for primary treatment. What is the daily dose in mg, and how is it built from 400 mg and 100 mg tablets?

Show the answer

Answer: 900 mg once daily; two 400 mg tablets plus one 100 mg tablet

Working: 60 kg x 15 mg = 900 mg. Using the larger strength first, two 400 mg tablets give 800 mg and one 100 mg tablet makes up the remaining 100 mg, so three tablets in total.

Question 2

A fictional adult weighs 80 kg and starts re-treatment with ethambutol at 25 mg per kg daily for the first 60 days. What is the daily dose in mg, and how many 400 mg tablets does it need?

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Answer: 2,000 mg once daily; five 400 mg tablets

Working: 80 kg x 25 mg = 2,000 mg. Five 400 mg tablets give exactly 2,000 mg, so no 100 mg tablets are needed.

Question 3

The same fictional 80 kg adult passes day 60 of re-treatment. What daily dose applies from day 61, and how many 400 mg tablets does it use?

Show the answer

Answer: 1,200 mg once daily; three 400 mg tablets

Working: 80 kg x 15 mg = 1,200 mg. Three 400 mg tablets give exactly 1,200 mg, so three tablets in total.

eMC SmPC: Ethambutol 400 mg Tablets

https://www.medicines.org.uk/emc/product/9346/smpc

Where marks are lost

  • Keeping the 25 mg per kg re-treatment dose beyond day 60 instead of reducing to 15 mg per kg from day 61.
  • Building the dose from 100 mg tablets first and ending with a larger tablet count than needed.
  • Dividing the daily dose across the day when the product information gives it as a single daily dose.

Frequently asked questions

Are these official assessment questions?

No. They are original revision exercises, not official GPhC questions and not for patient care.

Does this calculation decide a real dose?

No. Follow the stated prescription in the exercise. Real dosing follows the prescriber and the product information for the named product.

How should I check the answer?

Multiply the weight by the stated rate (15 mg per kg, or 25 mg per kg for the first 60 days of re-treatment), then build the answer from 400 mg and 100 mg tablets starting with the larger strength.

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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More calculation topics

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