GlossaryPharmacokineticsBioavailability
Pharmacokinetics

Bioavailability

Also known as: F · systemic availability

Bioavailability (F) is the fraction of a dose that reaches your bloodstream as intact drug, compared with the same dose given intravenously.

Bioavailability, written F, answers a simple question: of the amount you administered, how much actually reached your systemic circulation as intact drug? An intravenous dose goes straight into the blood, so it is the reference point, with F = 100% by definition. Every other route can lose drug before it gets there, whether it is broken down at the site, never crosses into the blood, or is destroyed on the way.

It is measured by comparing total exposure (AUC) after the route being tested with AUC after an intravenous dose, each adjusted for the dose given. That comparison is called absolute bioavailability; comparing two non-intravenous products or routes with each other gives relative bioavailability. Bioavailability tells you how much gets in, not how fast. Two products with the same F can still produce very different peaks if one is absorbed more slowly.

Why it matters

The same molecule can have very different bioavailability by route. Semaglutide injected SubQ has an absolute bioavailability of 89%. The same molecule as an oral tablet, even co-formulated with an absorption enhancer (SNAC), has an estimated absolute bioavailability of about 0.4% to 1% for the Rybelsus doses, which is why milligram amounts on an oral label cannot be compared one to one with an injection.

Bioavailability also differs between species, so animal numbers do not transfer. In the published BPC-157 study, intramuscular bioavailability was about 14% to 19% in rats and 45% to 51% in beagle dogs. That study gives no human figure for any route, so claims that oral, nasal or injected BPC-157 is absorbed well in people need their own human data.

How to calculate it

  1. Take the AUC after an intravenous dose and the AUC after the route being tested (these come from a pharmacokinetic study, not something you can measure at home).
  2. Divide each AUC by the dose that produced it.
  3. Divide the tested route's dose-adjusted AUC by the intravenous one; the result is F.
  4. To estimate how much drug reached circulation, multiply the dose by F.

F = (AUC_route / Dose_route) ÷ (AUC_IV / Dose_IV); amount reaching circulation = F × Dose

Worked example

Hypothetical study: 1 mg intravenous gives an AUC of 1,000 ng·h/mL; 2 mg SubQ gives 1,100 ng·h/mL. F = (1,100 / 2) ÷ (1,000 / 1) = 550 ÷ 1,000 = 0.55, or 55%. With semaglutide label figures: a 1 mg SubQ dose × 0.89 = 0.89 mg reaches circulation, while a 14 mg tablet × 0.004 to 0.01 = 0.056 to 0.14 mg does.

Common mistakes

  • Assuming every SubQ peptide is absorbed almost completely; bioavailability is compound-specific and has to be measured.
  • Using an animal bioavailability value for people (BPC-157's intramuscular F was about 14% to 19% in rats but 45% to 51% in dogs).
  • Treating an oral or nasal peptide product as equivalent to an injection without human bioavailability data for that exact product.
  • Confusing bioavailability (how much gets in) with absorption rate (how fast it gets in).
  • Comparing milligram doses across routes without adjusting for F.

Sources

  1. Prescribing informationOZEMPIC (semaglutide) injection, for subcutaneous use: Prescribing Information. Novo Nordisk (FDA-approved labeling), 2026.
  2. Prescribing informationRYBELSUS (semaglutide) tablets and OZEMPIC (semaglutide) tablets, for oral use: Prescribing Information. Novo Nordisk (FDA-approved labeling), 2026.
  3. Preclinical studyPharmacokinetics, distribution, metabolism, and excretion of body-protective compound 157, a potential drug for treating various wounds, in rats and dogs. Front Pharmacol, 2022. PMID 36588717.

Page updated . How we grade evidence: methodology.

Related Terms

Subcutaneous Absorption Rate

Subcutaneous absorption rate is how fast a drug injected into the fatty layer under your skin moves into your bloodstream.

AUC (Area Under Curve)

AUC (area under the curve) is your total exposure to a drug over time: the area under its blood concentration-time curve.

Clearance Rate

Clearance (CL) is the volume of plasma your body fully clears of a drug per unit of time, usually given in liters per hour.

Volume of Distribution

Volume of distribution (Vd) is the volume a drug would have to fill, at its plasma concentration, to account for all the drug in your body.

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