Common Registration Assessment, Part 1

Permethrin cream quantities and whole-body applications for the GPhC assessment

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

Published 10 November 2026. Independent revision material, not GPhC questions, not for patient care.

A person applying white cream to their forearm
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 permethrin 5% w/w cream states that it is intended for a single application over the whole body, applied to clean, dry, cool skin, and that the whole body should be washed thoroughly 8 to 12 hours after application.

The same information gives quantities by age: adults, the elderly and adolescents over 12 years apply up to 30 g (one 30 g tube), and a few adults may need an additional tube but should not use more than 60 g at each application; children 6 to 12 years apply up to 15 g (half a tube); children 1 to 5 years up to 7.5 g (a quarter of a tube); and children 2 months to 1 year up to 3.75 g (an eighth of a tube).

It also states that the cream should be reapplied to the hands if they are washed within 8 hours of treatment. A 5% w/w cream contains 5 g of permethrin in 100 g of cream, which is 50 mg per gram.

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

The method

  1. Match the stated patient to the stated maximum quantity in grams and its fraction of a 30 g tube.
  2. Check any adult quantity against the stated 60 g per application ceiling.
  3. Using 50 mg of permethrin per gram of cream, multiply grams by 50 for the mg content.
  4. Keep grams of cream, mg of permethrin, tubes and age bands 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 is treated with permethrin 5% cream. What is the label maximum quantity for one application, including the case where a second tube is needed?

Show the answer

Answer: Up to 30 g normally; no more than 60 g at each application

Working: The label states adults apply up to 30 g (one tube), and a few adults may need an additional tube but should not use more than 2 tubes (60 g in total) at each application.

Question 2

A fictional 8-year-old child is treated with permethrin 5% cream. What is the label maximum quantity, and what fraction of a 30 g tube is it?

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Answer: 15 g; half a tube

Working: Children 6 to 12 years apply up to 15 g, which the label equates to half a 30 g tube.

Question 3

A fictional adult applies 30 g of permethrin 5% w/w cream. Using 50 mg of permethrin per gram of cream, how many mg of permethrin are applied?

Show the answer

Answer: 1500 mg

Working: 5% w/w = 5 g per 100 g = 50 mg per g. 30 g x 50 mg = 1500 mg of permethrin.

eMC SmPC: Permethrin 5% w/w Cream

https://www.medicines.org.uk/emc/product/6540/smpc

Where marks are lost

  • Giving a child the adult 30 g quantity; the label scales down to 15 g, 7.5 g and 3.75 g by age band.
  • Treating 60 g as the usual adult quantity; 30 g is the usual maximum and 60 g is the absolute ceiling when a second tube is needed.
  • Reporting grams of cream when the question asks for mg of permethrin; convert with 50 mg of permethrin per gram of cream.

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 permethrin quantity?

No. Follow the stated patient in the exercise. Real treatment is a single whole-body application matched to the age band, washed off after 8 to 12 hours.

How should I check the answer?

Fix the age-band quantity in grams first, then convert to mg of permethrin at 50 mg per gram only when asked.

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.