Bioavailability deals with the extent and rate of drug absorption.
Route of Administration
Absorption
intravascular
immediate and complete
oral
delayed and often incomplete
intramuscular
delayed and potentially incomplete
F = fraction of oral dose that reaches the systemic circulation =
= (amount of dose reaching systemic circulation) / (amount of drug administered)
Factors reducing oral absorption include (see next section):
(1) food
(2) altered blood flow
(3) disease
(4) other medications
(5) rate and completeness of release for the drug from its delivery vehicle
Orally administered drugs enter the portal venous system and pass through the liver. A high rate of hepatic metabolism (first pass effect) can significantly reduce the amount of drug that reaches the systemic circulation.
The relative bioavailability of dosage forms for a drug (or routes of administration) can be calculated based on the different doses and areas under the drug concentration curve (AUC).
relative bioavailability for A vs B =
= ((AUC for A) / (AUC for B)) * ((dose for B) / (dose for A)) =
= (fraction of dose A absorbed) / (fraction of dose B absorbed)
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