Common Registration Assessment, Part 1

Erythromycin 250 mg and 500 mg tablets and daily totals for the GPhC assessment

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

Published 2 November 2026. Independent revision material, not GPhC questions, not for patient care.

White tablets in a blister pack on an orange background
Photo from Pexels, used under the Pexels licence. It is a stock image and does not show the exercise.

The NHS describes erythromycin as an antibiotic used for bacterial infections, usually taken 2 to 4 times a day for a few days or weeks.

The manufacturer's UK product information lists 250 mg and 500 mg tablets, with a usual adult dose of 1 g to 2 g a day in divided doses for mild to moderate infections.

The same product information allows up to 4 g a day in divided doses for severe infections.

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 strength and frequency stated in the question.
  2. Multiply the single dose by the stated doses per day to get the daily total.
  3. Convert grams into milligrams before counting tablets.
  4. Multiply the daily tablet count by the stated number of days.
  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 takes erythromycin 500 mg four times a day. How many milligrams a day is that, and how many 500 mg tablets?

Show the answer

Answer: 2,000 mg a day; 4 tablets

Working: 500 mg x 4 doses = 2,000 mg; 4 doses x 1 tablet = 4 tablets a day.

Question 2

A fictional prescription reads erythromycin 500 mg four times a day for 7 days. How many tablets should be supplied, and how many milligrams is the course?

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Answer: 28 tablets; 14,000 mg

Working: 4 tablets x 7 days = 28 tablets; 2,000 mg x 7 days = 14,000 mg.

Question 3

A fictional patient with a severe infection takes 1 g four times a day using 500 mg tablets. How many tablets is each day, and is this within the stated maximum daily amount?

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Answer: 8 tablets a day; yes, 4 g equals the stated maximum

Working: 1 g = 1,000 mg; 1,000 mg / 500 mg = 2 tablets per dose; 2 tablets x 4 doses = 8 tablets a day; 1 g x 4 doses = 4 g, the stated maximum daily amount.

NHS: Erythromycin

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

Where marks are lost

  • Forgetting to convert grams into milligrams before counting tablets.
  • Reporting the single dose instead of the daily total.
  • 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 dose-total calculation justify changing a real dose?

No. Follow the steps in the exercise. Real dose changes are gradual and 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.

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More calculation topics

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