Bioavailability is described as

• A. The fraction of administered dose that reaches systemic circulation
• B. What is left of drug after swallowing. PO meds will never have 100% bioavailability
• C. The rate at which a drug is eliminated from the body
• D. The maximum concentration achievable in plasma

Answer: (B)

Bioavailability is the fraction of the administered dose that reaches systemic circulation, regardless of the route. It tells you how much of the drug actually becomes available in the blood to exert an effect. For oral medicines, this is often less than 100% because some drug is lost to incomplete absorption and because of first-pass metabolism in the gut wall and liver before it enters the bloodstream.

This concept matters because it helps determine the true amount of drug that can act systemically and informs dosing to achieve the desired plasma levels. The other descriptions don’t define bioavailability: one describes what remains after swallowing but doesn’t quantify what reaches the blood; another refers to the rate of elimination (clearance), not the amount that reaches systemic circulation; and the last describes the maximum plasma concentration, which relates to dose and absorption kinetics but not the overall fraction that becomes bioavailable.