Common Registration Assessment, Part 1

Phenytoin capsule counts and mg per kg daily doses for the GPhC assessment

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

Published 3 November 2026. Independent revision material, not GPhC questions, not for patient care.

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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 Phenytoin Sodium Flynn Hard Capsules recommends an initial adult dose for seizures of 3 to 4 mg/kg/day, with later adjustment if needed.

The same information states that for most adults a satisfactory maintenance dose will be 200 to 500 mg daily, given in single or divided doses.

It also warns that a period of seven to ten days may be required to achieve steady state serum levels, and that changes in dosage should not be carried out at intervals shorter than seven to ten days.

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 a supply period. All prescriptions and patients here are fictional; these are revision materials, not for patient care.

The method

  1. Multiply the stated body weight by the stated mg per kg figure to get the daily dose.
  2. Divide the daily dose by the capsule strength to count capsules per day.
  3. Multiply the daily capsule count by the stated number of days for a supply quantity.
  4. Keep milligrams and capsules 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 patient weighs 75 kg and starts phenytoin at 4 mg/kg/day. How many milligrams is the daily dose?

Show the answer

Answer: 300 mg

Working: 75 kg x 4 mg/kg = 300 mg each day.

Question 2

A fictional prescription reads phenytoin 400 mg daily using 100 mg capsules. How many capsules are taken each day?

Show the answer

Answer: 4 capsules

Working: 400 mg / 100 mg = 4 capsules a day.

Question 3

A fictional patient takes phenytoin 300 mg daily using 100 mg capsules. How many capsules are needed for 28 days?

Show the answer

Answer: 84 capsules

Working: 300 mg / 100 mg = 3 capsules a day; 3 capsules x 28 days = 84 capsules.

eMC SmPC: Phenytoin Sodium Flynn Hard Capsules 100mg

https://www.medicines.org.uk/emc/product/7558/smpc

Where marks are lost

  • Forgetting to multiply the weight by the mg per kg figure before counting capsules.
  • Rounding capsule counts mid-calculation instead of checking the total against the stated dose.
  • 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 for phenytoin are gradual, use serum level monitoring, 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.