Principles of pharmacology and pharmacokinetics - bioavailability

Definition

Bioavailability is the fraction of an administered drug dose that reaches systemic circulation unchanged - 100% by definition for intravenous administration, and typically reduced for oral administration through incomplete absorption and hepatic first-pass metabolism before the drug reaches systemic circulation
A foundational pharmacokinetic parameter relevant to every orally administered medication, directly affecting the relationship between prescribed dose and actual systemic drug exposure

First-pass hepatic metabolism - oral drugs absorbed from the gut are carried via the portal vein directly to the liver before reaching systemic circulation, where extensive metabolism can substantially reduce the amount of active drug reaching the rest of the body - explaining why some drugs (e.g. glyceryl trinitrate) have negligible oral bioavailability and require alternative routes (sublingual, transdermal) that bypass first-pass metabolism

Not directly measured in routine clinical practice, but understanding a specific drug's bioavailability explains dose differences required between routes of administration for the same drug (e.g. oral vs IV opioid dose conversion)

Route selection should account for bioavailability differences - switching a patient between oral and IV formulations of the same drug requires dose adjustment reflecting the bioavailability difference, not a simple milligram-for-milligram substitution
Drugs with low and variable oral bioavailability may be preferentially given by a route bypassing first-pass metabolism when reliable, predictable dosing is required

Oral-to-IV opioid dose conversion (a practical, frequently applied application), sublingual/transdermal routes bypassing first-pass metabolism, hepatic impairment further reducing first-pass clearance and increasing effective bioavailability of some drugs
Switching a patient's medication between oral and intravenous formulations requires a bioavailability-informed dose adjustment, not a simple milligram-for-milligram substitution - failing to account for this difference is a recognised source of prescribing error, particularly relevant to opioid dose conversion in pain and palliative care management

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