Common Registration Assessment, Part 1

Rifampicin capsule doses by body-weight band for the GPhC assessment

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

Published 12 November 2026. Independent revision material, not GPhC questions, not for patient care.

Orange soft capsules scattered on a pink background
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 Rimactane 300 mg capsules states that the adult dose of rifampicin is 450 to 600 mg once daily, guided by about 10 mg per kg of body weight.

The same information places patients weighing 50 kg and over on 600 mg once daily and patients under 50 kg on 450 mg once daily.

The product information states that rifampicin is always given as part of combination therapy for tuberculosis, and that one capsule contains 300 mg of rifampicin.

Each question fixes the values needed for its arithmetic. Write the unit beside every step and keep the weight band beside the dose it selects. All prescriptions and patients here are fictional; these are revision materials, not for patient care.

The method

  1. Compare the stated body weight with the 50 kg boundary in the product information.
  2. Select 600 mg for 50 kg and over, or 450 mg for under 50 kg.
  3. Check the selection against about 10 mg per kg where the question asks.
  4. Divide a stated milligram dose by 300 mg for the capsule count where it divides exactly.
  5. Multiply the once-daily capsule count by the stated days for a supply.

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 52 kg and is prescribed rifampicin once daily at the product-information dose for their weight band. Using 300 mg capsules, how many capsules is each daily dose?

Show the answer

Answer: 2 capsules

Working: 52 kg is in the 50 kg and over band, so the dose is 600 mg once daily; 600 mg / 300 mg = 2 capsules.

Question 2

A fictional adult weighs 45 kg. What once-daily dose does the weight band select, and what does a check at about 10 mg per kg give?

Show the answer

Answer: 450 mg once daily; the check also gives about 450 mg

Working: 45 kg is under 50 kg, so the band selects 450 mg once daily; 45 kg x 10 mg = 450 mg, which matches.

Question 3

A fictional adult weighing 70 kg takes rifampicin 600 mg once daily using 300 mg capsules. How many capsules does a 30-day supply need?

Show the answer

Answer: 60 capsules

Working: 600 mg / 300 mg = 2 capsules a day; 2 capsules x 30 days = 60 capsules.

eMC SmPC: Rimactane Capsules 300mg

https://www.medicines.org.uk/emc/product/4001/smpc

Where marks are lost

  • Placing a patient of exactly 50 kg in the lower band; 50 kg and over takes 600 mg.
  • Splitting the once-daily dose into two doses when the stated schedule is once daily.
  • Forcing a capsule count from a dose that does not divide exactly by the 300 mg strength.

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 rifampicin dose?

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

How should I check the answer?

Fix the weight against the 50 kg boundary, read off 450 mg or 600 mg once daily, then divide by 300 mg for capsules only where it divides exactly.

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.