Which term indicates the fraction of an administered dose that reaches systemic circulation?

• A. The fraction of administered dose that reaches systemic circulation
• B. The time to onset of action
• C. The rate of metabolism
• D. The distribution volume

Answer: (A)

Bioavailability is the fraction of an administered dose that reaches systemic circulation. It describes how much of the drug becomes available in the bloodstream to have an effect after a given route of administration. For IV administration, bioavailability is effectively 100% because the drug enters the circulation directly; for other routes, it’s often lower due to incomplete absorption and first-pass metabolism. The other terms describe different pharmacokinetic concepts: time to onset of action is when effects begin, rate of metabolism is how quickly the body breaks down the drug, and distribution volume relates to how widely the drug distributes between plasma and tissues. Understanding bioavailability helps explain why dosing may differ between routes and formulations.