Common Registration Assessment, Part 1

Co-trimoxazole tablet strengths and twelve-hourly course counts for the GPhC assessment

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

Published 11 November 2026. Independent revision material, not GPhC questions, not for patient care.

A hand holding several white round tablets
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 Co-Trimoxazole 80mg/400mg Tablets states that the standard adult dose is 2 tablets every 12 hours, that treatment should be continued until the patient has been symptom free for two days, that the majority will require treatment for at least 5 days, and that the patient should be reassessed if clinical improvement is not evident after 7 days.

The same information states that where dosage is expressed as tablets it refers to the adult tablet of 80 mg trimethoprim with 400 mg sulfamethoxazole, and that the standard dosage for children is approximately 6 mg trimethoprim and 30 mg sulfamethoxazole per kg body weight per day in two equally divided doses.

The Forte tablet contains double each component (160 mg trimethoprim with 800 mg sulfamethoxazole), dosed at 1 tablet every 12 hours, so one Forte tablet replaces two standard tablets.

Each question fixes the values needed for its arithmetic. Write the unit beside every step and distinguish the standard tablet from the Forte tablet. All prescriptions and patients here are fictional; these are revision materials, not for patient care.

The method

  1. Identify whether the stated product is the standard 80mg/400mg tablet or the Forte 160mg/800mg tablet.
  2. Multiply the stated tablets per dose by two for the twelve-hourly daily count.
  3. Multiply the daily count by the stated days for a course quantity.
  4. Keep tablets, mg of trimethoprim and mg of sulfamethoxazole 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 takes co-trimoxazole standard 80mg/400mg tablets at the label dose for a 5-day course. How many tablets are taken each day, and how many over the course?

Show the answer

Answer: 4 tablets daily; 20 tablets

Working: The label dose is 2 tablets every 12 hours. 2 x 2 = 4 tablets daily. 4 x 5 days = 20 tablets.

Question 2

A fictional adult is switched from co-trimoxazole standard tablets to the Forte 160mg/800mg tablet for the same label schedule. How many Forte tablets are taken each day, and how does each Forte tablet compare with the standard tablet?

Show the answer

Answer: 2 Forte tablets daily; one Forte tablet equals two standard tablets

Working: The Forte dose is 1 tablet every 12 hours: 1 x 2 = 2 tablets daily. The Forte tablet holds 160 mg trimethoprim with 800 mg sulfamethoxazole, exactly twice the standard 80 mg/400 mg tablet.

Question 3

A fictional adult takes one dose of 2 standard co-trimoxazole 80mg/400mg tablets. How many mg of trimethoprim and of sulfamethoxazole are in that single dose?

Show the answer

Answer: 160 mg trimethoprim and 800 mg sulfamethoxazole

Working: 2 x 80 mg = 160 mg trimethoprim. 2 x 400 mg = 800 mg sulfamethoxazole. Over a day at the twelve-hourly schedule that is 320 mg and 1600 mg.

eMC SmPC: Co-Trimoxazole 80/400mg Tablets

https://www.medicines.org.uk/emc/product/11465/smpc

Where marks are lost

  • Mixing up the standard tablet and the Forte tablet; one Forte 160 mg/800 mg tablet replaces two standard 80 mg/400 mg tablets.
  • Counting the dose instead of the day; the label schedule is every 12 hours, so the daily count is twice the per-dose count.
  • Adding trimethoprim and sulfamethoxazole into a single mg figure; each component keeps its own total.

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 co-trimoxazole course?

No. Follow the stated prescription in the exercise. Real courses follow the prescriber and the product information for the named tablet.

How should I check the answer?

Name the tablet strength first, multiply the per-dose count by two for the twelve-hourly day, then multiply by the stated days.

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.