Common Registration Assessment, Part 1

Methadone liquid daily doses and millilitre volumes for the GPhC assessment

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

Published 31 October 2026. Independent revision material, not GPhC questions, not for patient care.

Gloved hand holding an orange prescription pill bottle
Photo from Pexels, used under the Pexels licence. It is a stock image and does not show the exercise.

The NHS states that the usual starting dose of methadone is 10 mg to 30 mg taken once a day, and that many people then take a regular dose of between 60 mg and 120 mg a day.

The BNF states that the dose is increased in steps of 5 mg to 10 mg, with a maximum weekly dose increase of 30 mg, and, for Methadose, that the final strength of the mixture to be dispensed must be specified on the prescription.

The NHS states that methadone for heroin dependency usually comes as a green liquid swallowed once a day, that early doses are usually supervised at a pharmacy, and that detox courses reduce the dose gradually over up to 12 weeks.

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 stated daily dose in milligrams and the stated solution strength.
  2. Divide the dose in milligrams by the strength in milligrams per millilitre to get the volume per dose.
  3. Multiply the daily volume by the number of days for a supply.
  4. Check any dose increase against the stated weekly cap before accepting it.
  5. Keep milligrams and millilitres separate; convert only with the stated strength.

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 methadone 80 mg once a day using an oral solution containing 1 mg in 1 ml. How many millilitres is each daily dose, and how many millilitres for 7 days?

Show the answer

Answer: 80 ml per day; 560 ml for 7 days

Working: 80 mg / 1 mg per ml = 80 ml per day; 80 ml x 7 days = 560 ml.

Question 2

A fictional dose starts at 20 mg once a day, rises by 10 mg at the start of week 2, and by a further 10 mg at the start of week 3. What is the daily dose in week 3, and is each weekly increase within a 30 mg weekly cap?

Show the answer

Answer: 40 mg once a day; yes, each increase is 10 mg, below the 30 mg cap

Working: 20 mg + 10 mg = 30 mg in week 2; 30 mg + 10 mg = 40 mg in week 3; 10 mg is less than 30 mg.

Question 3

A fictional pharmacy supervises doses Monday to Saturday and supplies Sunday's dose to take at home. For a patient on 60 mg daily using a solution containing 1 mg in 1 ml, how many millilitres leave the pharmacy as Sunday doses over 4 weeks?

Show the answer

Answer: 240 ml

Working: 60 mg / 1 mg per ml = 60 ml per Sunday; 60 ml x 4 Sundays = 240 ml.

NHS: How and when to take methadone

https://www.nhs.uk/medicines/methadone/how-and-when-to-take-methadone/

Where marks are lost

  • Confusing milligrams with millilitres when the strength is stated as 1 mg in 1 ml.
  • Adding more than the stated weekly increase in one step.
  • 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.

Does a volume calculation apply to any methadone liquid?

Only when the strength is stated, as in these exercises. Real prescriptions specify the mixture strength to dispense.

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.

Practise until the method is automatic

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More calculation topics

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