Common Registration Assessment, Part 1
Half-life and falling drug levels
Each half-life halves the level. Three original questions on how long a drug level takes to fall.
Published 6 October 2026. Independent revision material, not GPhC questions, not for patient care.
The Pharmaceutical Journal's pharmacokinetics article defines the elimination half-life as the time it takes for the concentration of a drug to fall to half. It also makes the point that patients have half-lives, not drugs: gentamicin's half-life is two to three hours in a young adult with normal kidney function and 24 hours or more in severe renal impairment.
For exam questions the working is usually a halving sequence. Count the half-lives, then multiply by the length of one.
The method
- Write the starting level and the target level.
- Halve the level repeatedly until you reach or pass the target, counting the halvings.
- Multiply the number of half-lives by the half-life in hours.
- Check the answer against the patient's situation, for example renal impairment.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
A drug has a half-life of 6 hours. The level is 40 mg/L now. What will the level be after 18 hours? These figures are made up for the exercise.
Show the answer
Answer: 5 mg/L
Working: 18 ÷ 6 = 3 half-lives. 40 → 20 → 10 → 5 mg/L.
Question 2
A drug level is 64 mg/L and the half-life is 5 hours. How long will it take for the level to fall to 4 mg/L? These figures are made up for the exercise.
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Answer: 20 hours
Working: 64 → 32 → 16 → 8 → 4 is 4 half-lives. 4 × 5 = 20 hours.
Question 3
A level of 12 mg/L is measured in a patient whose half-life is 24 hours because of severe renal impairment. How long until it falls to 3 mg/L, and how long would it take with a half-life of 3 hours? These figures are made up for the exercise.
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Answer: 48 hours; 6 hours
Working: 12 → 6 → 3 is 2 half-lives. 2 × 24 = 48 hours with impaired kidney function, against 2 × 3 = 6 hours with a 3-hour half-life. The same two half-lives take very different times.
What the article says
The article defines the half-life and uses gentamicin to show how renal impairment lengthens it. The levels and half-lives on this page are invented for practice.
Where marks are lost
- Dividing the level by the half-life in hours instead of halving it.
- Counting the starting level as a half-life.
- Forgetting that a patient's half-life can be much longer than the typical figure.
Frequently asked questions
Is the half-life fixed for a drug?
Not for a patient. The article notes that it depends on clearance and volume of distribution, which change with illness.
How many half-lives to get close to nothing?
Questions usually give a target level, so count halvings to that level.
Sources
- The Pharmaceutical Journal, back to basics: pharmacokinetics
- GPhC, Common Registration Assessment specification and permitted items for 2026
The questions above are original and use fictional drugs. 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.
More calculation topics
- Dose by weight calculations for the GPhC assessment
- Volume to give: liquid medicine calculations
- Percentage strength calculations: w/v, w/w and ratios
- Dilution calculations with C1V1 = C2V2
- Infusion rate calculations in mL per hour
- Drip rate calculations: drops per minute
- Body surface area calculations
- Creatinine clearance (Cockcroft-Gault) calculations
- Moles and millimoles calculations
- Displacement volume calculations for reconstitution
- Quantity to supply calculations
- Using a provided formula in calculations
- Dose information from packaging and labels
- Diluting a stock to a lower strength
- Medicine cost and switch calculations
- Rounding rules in calculations
- Enteral feed rate calculations
- Electrolyte content over time
- Infusion dose per kg per hour
- Rounding doses to measurable volumes
- Relative risk and relative risk reduction
- Absolute risk reduction and number needed to treat
- Odds ratio and confidence intervals
- Relative versus absolute risk reduction
- Loading dose and volume of distribution
- Top-up doses from measured drug levels
- Salt correction factors
- Mass and molar drug concentrations
- Steady-state concentration and infusion rate
See also the formula sheet, the eight sample questions and the approved calculator page.