Learning Objective

By the end of this section, the learner will be able to define bioavailability, explain how it is measured, compare intravascular and extravascular administration, and interpret differences in AUC (area under the plasma concentration–time curve) for various routes of drug delivery.

Bioavailability (f)

Bioavailability (f) refers to the fraction of an administered dose of drug that reaches the systemic circulation in its active form. It is determined by comparing the area under the curve (AUC) of a drug given extravascularly (e.g., oral) with the AUC of the same drug given intravascularly.

Intravascular administration (IV):

• No absorption phase
• No loss of drug
• Bioavailability = 100% → f = 1

Extravascular administration (e.g., PO, IM, SC):

• A drug may be incompletely absorbed or undergo first-pass metabolism
• Bioavailability < 100%

Calculation:

​​A larger AUC reflects a greater amount of drug reaching the bloodstream.

Activity