Common Registration Assessment, Part 1

Gentamicin weight-based daily doses and injection volumes for the GPhC assessment

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

Published 9 November 2026. Independent revision material, not GPhC questions, not for patient care.

A row of clear unlabeled medicine vials
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 Cidomycin 80 mg/2 ml solution for injection states that the recommended dose in adults with normal renal function is 3 to 5 mg/kg/day, depending on the severity of infection, administered as one single dose (preferred) or in two divided doses, and that dose calculations should be based on ideal body weight.

The same information states that in patients with normal renal function, 160 mg once daily may be used for the treatment of urinary tract infections, and that the daily dose recommended in children aged 1 year and above with normal renal function is 3 to 6 mg/kg/day as one single dose (preferred) or two divided doses.

Cidomycin is supplied as 80 mg in 2 ml, which is 40 mg per ml. Once-daily dosing is not recommended in cases of endocarditis, depending on the responsible pathogens, and national and local guidance on treatment and serum level monitoring should be followed.

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

The method

  1. Multiply the stated mg/kg by the stated body weight for the daily dose in mg.
  2. If the dose is divided, split the daily total into the stated number of administrations.
  3. Divide the dose in mg by the 40 mg per ml concentration for the injection volume.
  4. Keep mg/kg, mg, ml and ampoule counts 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 weighs 70 kg and is prescribed gentamicin 5 mg/kg/day as one single daily dose. Using ideal body weight as stated, what is the daily dose, and what volume of Cidomycin 80 mg/2 ml (40 mg per ml) does it need?

Show the answer

Answer: 350 mg; 8.75 ml

Working: 5 mg/kg x 70 kg = 350 mg daily. Volume: 350 mg / 40 mg per ml = 8.75 ml.

Question 2

A fictional adult weighs 60 kg and is prescribed gentamicin 3 mg/kg/day in two divided doses. What is the daily dose, and how much is given per administration?

Show the answer

Answer: 180 mg daily; 90 mg per administration

Working: 3 mg/kg x 60 kg = 180 mg daily. Two divided doses: 180 mg / 2 = 90 mg per administration.

Question 3

A fictional patient with normal renal function is prescribed the label once-daily gentamicin dose for a urinary tract infection. What is the dose, and how many 2 ml ampoules of Cidomycin 80 mg/2 ml does it need?

Show the answer

Answer: 160 mg; 2 ampoules

Working: The label states 160 mg once daily may be used for urinary tract infections. 160 mg / 80 mg per ampoule = 2 ampoules (4 ml).

eMC SmPC: Cidomycin 80mg/2ml Solution for Injection

https://www.medicines.org.uk/emc/product/14742/smpc

Where marks are lost

  • Multiplying mg/kg by weight and reporting it as the per-administration dose when the daily dose is given in two divided doses.
  • Dividing the dose by 80 mg per ampoule when the question asks for a volume; the volume comes from the 40 mg per ml concentration.
  • Using actual body weight when the label states dose calculations should be based on ideal body weight.

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 gentamicin dose?

No. Follow the stated prescription in the exercise. Real dosing is adjusted to clinical response and serum concentrations, with renal function taken into account.

How should I check the answer?

Multiply mg/kg by the stated weight first, then divide mg by 40 mg per ml for the volume, keeping the daily dose and each administration separate.

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.