BMS 123 · Common Core (Tronc commun)Section 1.5 · Pharmacokinetics and drug absorption
Section 5 of 27
Bioavailability and bioequivalence
Define bioavailability, explain how AUC and Cmax measure extent and rate, calculate and interpret the bioavailability factor F, and explain when two products are bioequivalent and therapeutically equivalent.
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Why this matters
Absorption only matters if enough drug actually arrives, and bioavailability is the number that says how much. It explains why an injected dose and a swallowed dose of the same drug are not interchangeable, why the same product can behave differently in two patients, and why a generic tablet can be approved without repeating every clinical trial. The bioavailability factor turns all the absorption factors of the previous sections into one measurable quantity, and bioequivalence turns that quantity into a rule for deciding whether two products may be used in place of each other.
What you need to know first
You need Units 1.3 and 1.4 for the factors that change absorption, and the simple idea of plotting a concentration against time and measuring the area beneath the curve.
Words to know (5)
Bioavailability
The extent and rate at which the active ingredient of a drug product is absorbed and becomes available at the site of drug action. A drug with low bioavailability must be given in a larger oral dose to produce the same effect.
Area under the curve (AUC)
The area beneath the plasma concentration–time curve, which measures the total extent of absorption of the drug. A larger AUC means more drug reached the blood over the period measured.
Cmax
The highest plasma concentration reached after a dose, which reflects how fast the drug was absorbed. A rapidly absorbed drug produces an early, high Cmax.
Bioavailability factor (F)
The fraction of the dose that reaches the systemic circulation intact, ranging from zero to one. An F of 0.80 means that 80 percent of the dose reached the circulation as intact drug.
Bioequivalence
The condition in which the rate and extent of absorption of two products are not statistically different at the same molar dose under similar conditions. A generic tablet may be approved when it is shown to be bioequivalent to the original product.
One idea at a timeIdea 1 / 5
In one sentence Bioavailability is the extent and rate at which the active ingredient of a product is absorbed and becomes available at the site of action.
Bioavailability measures both extent and rate
Bioavailability connects absorption with the availability of the active ingredient at its site of action. It answers two practical questions: how much of the dose becomes available there, and how quickly? A product can deliver the same total amount as another and still differ clinically because it delivers it more slowly or more quickly. Bioavailability is therefore a property of the product given by a particular route in a particular patient, not a fixed label on the molecule. Solubility, ionisation, dosage form, route, blood flow, contact time and first-pass metabolism all change it.
For example
A drug given as a rapidly dissolving solution and the same drug given as a slow-release tablet may deliver a similar total amount, yet the solution produces an earlier and higher peak.
Pause and check yourself
Which two aspects does bioavailability measure?
Show the answer
The extent of absorption — how much of the dose arrives — and the rate of absorption — how quickly it arrives at the site of action.
Read detailed explanations
Reading a concentration–time curve properly
A plasma concentration–time curve contains more information than AUC and Cmax alone, and reading it well is the skill behind bioavailability studies. The curve rises as absorption outpaces elimination, reaches a peak when the two rates are equal, and then falls as elimination dominates. The height of the peak is Cmax and the time at which it occurs is Tmax, which is a direct expression of the rate of absorption: a product absorbed quickly peaks early and high, while a slow-release product peaks late and low. The area beneath the whole curve is the AUC, and it measures extent: it is the same for two products that deliver the same total amount to the blood, whatever their shapes. This is why a comparison of AUC alone can call two products equivalent in amount while they differ in rate. For a drug taken for a chronic condition, a difference in rate may matter little; for a drug expected to act quickly, such as a painkiller or a hypnotic, the same difference is clinically important. Reading extent and rate together is therefore not pedantry, it is the reason both are reported.
Why bioequivalence is a statistical judgement, not a chemical one
Two tablets can be chemically identical — same molecule, same dose, same purity — and still deliver the drug differently, because dissolution, excipients and manufacturing compress each tablet into a different physical product. Bioequivalence therefore cannot be decided by chemical analysis of the tablet; it must be decided by measuring what appears in the blood. The definition reflects this: the two products are bioequivalent when the rate and extent of absorption are not statistically different when given at the same molar dose under similar experimental conditions. Two phrases deserve attention. “Statistically different” means the comparison is made on groups of subjects with a stated level of confidence, not on a single pair of measurements, because biological measurements always scatter. “Similar experimental conditions” means the same kind of subjects, the same food state and the same schedule of blood sampling, so that the two products are compared fairly. When the comparison is passed, the assumption of therapeutic equivalence follows, and the products can be treated as interchangeable. The value of the whole procedure is that it substitutes a measurable blood exposure for an unmeasurable assumption of sameness.
Explore the diagram
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Plasma curves after intravenous and oral doses
The intravenous curve starts high and falls, while the oral curve rises to a lower peak and falls later; the area under each curve measures the extent of absorption.
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01 / 5Intravenous curve
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Intravenous curve
After an intravenous dose the whole amount is in the blood immediately, so the curve begins at its highest point and falls as the drug distributes and is eliminated, with no absorption phase at all.
Schematic curves, not measured data. Real curves differ with the drug, the formulation and the patient, and the vertical axis is deliberately unlabelled because the shapes matter more than the numbers here.
Key points to remember
✓Bioavailability is the extent and rate at which the active ingredient is absorbed and becomes available at the site of action.
✓The intravenous route is 100 percent bioavailable and serves as the reference for every other route.
✓AUC measures the extent of absorption; Cmax measures the rate.
✓The bioavailability factor F is the oral AUC divided by the intravenous AUC, and lies between zero and one.
✓An F of 0.80 means that 80 percent of the dose reached the circulation intact.
✓Two products are bioequivalent when their rate and extent of absorption are not statistically different, and they are then assumed therapeutically equivalent.
Go deeper
Worked example
Interpreting a bioavailability factor of 0.80. A new oral formulation of a drug is studied against the intravenous form. The intravenous AUC is 100 arbitrary units; the oral AUC is 80. The oral Cmax is lower and arrives later than the intravenous Cmax. You are asked to interpret these results and decide what they imply.
Calculate F: divide the oral AUC by the intravenous AUC, giving 80 divided by 100, so F equals 0.80.
Interpret the extent: 80 percent of the oral dose reached the general circulation intact, and the missing 20 percent was lost before arrival.
Interpret the rate: the lower and later Cmax shows that absorption was slower than the intravenous reference, which gave an immediate peak.
Locate the likely losses using earlier sections: incomplete dissolution, poor membrane crossing, destruction in the gut or first-pass metabolism in the liver.
An F of 0.80 says that four fifths of the swallowed dose reached the blood; the Cmax comparison adds that it arrived more slowly. Extent and rate together describe bioavailability, and either one can change a patient’s response.
Common misconception
A frequent beginner error is to treat bioavailability as a synonym for absorption. Absorption is the process of crossing into the blood; bioavailability is the measured result — how much of the dose arrived intact and how fast — and it can be reduced after absorption by first-pass metabolism as well.
Check your reasoning
Two tablets contain the same drug and the same dose. Tablet A has an F of 0.90 and tablet B an F of 0.45. What does this difference mean for the patient, and why can the two products not simply be swapped without checking?
Reveal an answer
For the same swallowed dose, tablet B delivers only about half as much intact drug to the general circulation as tablet A, so its blood levels will be lower and its effect weaker. Swapping them without checking means changing the exposure the patient actually receives, unless a bioequivalence study has shown that the rate and extent of absorption are not statistically different. This is exactly why bioequivalence testing is mandatory for generic approval: the label dose can be identical while the delivered dose is not.
Where this comes from
BMS 123 General Pharmacology II lecture notes — Part 1, Principles of Pharmacokinetics · Extent of drug absorption: bioavailability, bioavailability factor, bioequivalence and therapeutic equivalence
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About this course and its sources
Bilingual lessons organised from the supplied BMS 123 General Pharmacology II lecture notes (Principles of Pharmacokinetics and Principles of Pharmacodynamics). Diagrams are original educational schematics; pharmacology, nursing and French-language review are pending.