Common Registration Assessment, Part 1

Clindamycin capsule strengths and six-hourly course counts for the GPhC assessment

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

Published 9 November 2026. Independent revision material, not GPhC questions, not for patient care.

A person holding a blister pack of capsules at a table
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 clindamycin 150 mg capsules states that for moderately severe infection adults take 150 to 300 mg every six hours, and that for severe infection adults take 300 to 450 mg every six hours.

The same information states that clindamycin should be dosed based on total body weight regardless of obesity in children able to swallow capsules, at doses of 12 to 25 mg/kg/day six hourly depending on the severity of the infection, and that the use of whole capsules may not be suitable to provide the exact mg/kg doses required for children.

It also states that in cases of beta-haemolytic streptococcal infection, treatment should continue for at least 10 days, and that capsules should always be taken whole with a full glass of water and no less than 30 minutes before lying down.

Each question fixes the values needed for its arithmetic. Write the unit beside every step and distinguish a six-hourly dose from a daily total. All prescriptions and patients here are fictional; these are revision materials, not for patient care.

The method

  1. Identify whether the stated infection is moderately severe or severe for the dose range.
  2. Divide the stated six-hourly dose by the 150 mg capsule strength for capsules per dose.
  3. Multiply by four doses daily for the daily count, then by the stated days for a course total.
  4. Keep mg per dose, capsules, days and course totals separate; convert only when 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 adult with a moderately severe infection takes clindamycin 300 mg every six hours, using 150 mg capsules. How many capsules per dose, and how many each day?

Show the answer

Answer: 2 capsules per dose; 8 capsules daily

Working: 300 mg / 150 mg = 2 capsules per dose. Every six hours means 4 doses daily: 2 x 4 = 8 capsules daily (1200 mg).

Question 2

A fictional adult with a severe infection takes clindamycin 450 mg every six hours, using 150 mg capsules. What is the total daily dose in mg?

Show the answer

Answer: 1800 mg

Working: 450 mg x 4 doses daily = 1800 mg daily (12 capsules).

Question 3

A fictional adult with a beta-haemolytic streptococcal infection takes clindamycin 150 mg every six hours for the label minimum of 10 days, using 150 mg capsules. How many capsules are needed for the course?

Show the answer

Answer: 40 capsules

Working: 1 capsule per dose x 4 doses daily = 4 capsules daily. 4 x 10 days = 40 capsules.

eMC SmPC: Clindamycin 150 mg Capsules

https://www.medicines.org.uk/emc/product/100601/smpc

Where marks are lost

  • Treating every six hours as three doses daily instead of four.
  • Mixing the moderately severe range of 150 to 300 mg with the severe range of 300 to 450 mg.
  • Stopping a beta-haemolytic streptococcal course before the label minimum of 10 days.

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 clindamycin course?

No. Follow the stated prescription in the exercise. Real dosing follows infection severity within the label ranges.

How should I check the answer?

Fix the six-hourly dose first, multiply by four for the daily total, then by the stated days for the course.

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.