Common Registration Assessment, Part 1
Bioavailability and equivalent oral doses
Practise converting between administered dose and absorbed dose, with three fictional examples and worked answers.
Published 7 October 2026. Independent revision material, not GPhC questions, not for patient care.
An oral dose and the amount reaching the circulation are not the same thing. Bioavailability, written as F, is the fraction that reaches the systemic circulation. In these exercises, 80% means F = 0.8, not 80.
For the simplified model used here, absorbed amount = administered dose × F. Work backwards by dividing the target absorbed amount by F. This is arithmetic practice, not a route-switching recommendation. Real switches also depend on the drug, formulation, timing and clinical response.
The method
- Identify whether the question asks for administered or absorbed amount.
- Convert the percentage to a decimal fraction.
- Multiply dose by F to find absorbed amount.
- Divide the required absorbed amount by F to find an administered dose.
- For a switch, equate absorbed amounts only when the question explicitly permits that model.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
Drug X has oral bioavailability of 75%. A single oral dose is 240 mg. Using the stated model, how much reaches the systemic circulation?
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Answer: 180 mg
Working: F = 0.75. Absorbed amount = 240 × 0.75 = 180 mg.
Question 2
A fictional formulation has F = 0.6. What oral dose supplies an absorbed amount of 90 mg?
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Answer: 150 mg
Working: Administered dose = 90 ÷ 0.6 = 150 mg. Check: 150 × 0.6 = 90 mg.
Question 3
An exercise assumes a 40 mg intravenous dose has F = 1 and the equivalent oral dose has F = 0.5. What oral dose gives the same absorbed amount?
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Answer: 80 mg
Working: The intravenous amount is 40 × 1 = 40 mg. Oral dose = 40 ÷ 0.5 = 80 mg. This uses only the equivalence model supplied.
A useful backwards check
If F is below 1, the absorbed amount cannot exceed the administered dose in this simple model. After a backwards calculation, multiply your dose by F. The result should return to the target amount.
Where marks are lost
- Multiplying instead of dividing when finding the dose needed.
- Entering 75 rather than 0.75 for 75%.
- Treating a fictional bioavailability as a real prescribing conversion.
Frequently asked questions
Can I assume every oral medicine has F = 1?
No. Use the value in the question or its reference. A missing value is not permission to invent one.
Does a lower bioavailability mean a smaller oral dose?
For the same target absorbed amount in this model, a lower F means a larger administered dose.
Sources
- The Pharmaceutical Journal, back to basics: pharmacokinetics
- GPhC, 2026 assessment specification and permitted items
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
- Half-life and falling drug levels
- Steady-state concentration and infusion rate
- Oral maintenance dose and dosing interval
- Elimination rate constant and half-life
- Accumulation towards steady state
- Ratio strengths and one in X
- Serial dilution factors
- Mixing solutions of different strengths
- Tablet quantities for reducing regimens
- Equivalent doses from provided conversion tables
- Thresholds and dose-banding algorithms
See also the formula sheet, the eight sample questions and the approved calculator page.