What is bioavailability?

Bioavailability (denoted F) is the fraction of an administered dose that reaches systemic circulation in unchanged form. It is one of the most useful single numbers in pharmacokinetics because it lets you translate an administered dose into an effective systemic exposure.

An intravenous dose is 100% bioavailable by definition — it enters the bloodstream directly. A subcutaneous or intramuscular injection has bioavailability typically 60–90% for peptides that are stable in interstitial fluid — some fraction is degraded or bound before reaching circulation. An intranasal dose has bioavailability typically 5–20% for peptides — the nasal mucosa is more permeable than the gastrointestinal tract, but not as absorptive as an injection depot. An oral dose without engineering enhancement typically has bioavailability under 1% for peptides — the gastrointestinal tract is designed to break peptides down before absorption.

Bioavailability multiplies with dose to give effective exposure. A 10 mg oral dose at 1% bioavailability delivers about 100 μg to circulation; a 100 μg subcutaneous dose at 90% bioavailability delivers about 90 μg to circulation. These two doses produce roughly comparable systemic exposure despite the 100-fold difference in administered mass. This is the arithmetic behind formulation choices: if oral bioavailability is low but production is cheap, engineering large-dose oral formulations may be economically preferable to smaller-dose injections; if bioavailability is high, injection allows precise dose control.