Common Registration Assessment, Part 1

Ethosuximide oral solution millilitre doses and titration steps for the GPhC assessment

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

Published 8 November 2026. Independent revision material, not GPhC questions, not for patient care.

A mother caring for her sick daughter on a sofa
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 ethosuximide 500 mg/5 ml oral solution states that for adults, elderly patients and children over 6 years of age, treatment is started at a daily dose of 500 mg.

The same information states that the dose is increased every five to seven days in increments of max 250 mg until seizures are controlled by a daily dose of 1000 to 1500 mg, and that in an individual case a daily dose of 2000 mg, taken in several single doses, may be required.

The solution contains 500 mg in each 5 ml, and higher daily doses should be taken in 2 or 3 single doses.

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

The method

  1. Identify the stated daily dose in mg and the solution strength, 500 mg in 5 ml.
  2. Divide the dose by 500 mg, then multiply by 5 ml, for the volume in ml.
  3. Check the stated titration steps: max 250 mg increments every five to seven days.
  4. Keep mg, ml and days 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 takes ethosuximide 500 mg daily using the 500 mg/5 ml oral solution. What volume is taken each day?

Show the answer

Answer: 5 ml

Working: 500 mg / 500 mg = 1, and 1 x 5 ml = 5 ml daily.

Question 2

A fictional patient takes ethosuximide 1500 mg daily using the 500 mg/5 ml oral solution. What volume is taken each day?

Show the answer

Answer: 15 ml

Working: 1500 mg / 500 mg = 3, and 3 x 5 ml = 15 ml daily.

Question 3

A fictional patient starts ethosuximide at 500 mg daily and the dose is increased by 250 mg every five days. What is the minimum number of days to reach a daily dose of 1000 mg?

Show the answer

Answer: 10 days

Working: From 500 mg to 1000 mg needs two 250 mg steps. 2 x 5 days = 10 days.

eMC SmPC: Ethosuximide Roma 500 mg/5 ml Oral solution

https://www.medicines.org.uk/emc/product/100383/smpc

Where marks are lost

  • Forgetting that 500 mg per 5 ml is 100 mg per ml before converting a dose to a volume.
  • Counting the starting dose as the first increment, so the days to reach a target are one step too many.
  • Mixing mg and ml in the same line of working.

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 ethosuximide dose?

No. Follow the stated prescription in the exercise. Real dosing is titrated to seizure control, with higher daily doses split into 2 or 3 single doses.

How should I check the answer?

Check the requested unit, then convert milligrams to millilitres using 500 mg per 5 ml before totalling.

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.