Bioavailability is defined as which of the following?

• A. The Time to Peak Concentration
• B. The Rate at Which the Drug Is Eliminated
• C. The Concentration at the Receptor Site
• D. The Fraction of Administered Drug That Reaches Systemic Circulation

Answer: (D)

Bioavailability tells us how much of an administered drug actually reaches the bloodstream in active form to have an effect. It’s defined as the fraction of the dose that reaches systemic circulation, often expressed as a percentage. This concept contrasts with how quickly the drug is absorbed (time to peak concentration) or how fast it is removed from the body (elimination rate). The concentration at the receptor site depends on distribution and pharmacodynamics, not just how much enters the bloodstream. For an intravenous dose, bioavailability is 100% because the drug is delivered directly into circulation. For oral or other routes, it’s reduced by incomplete absorption and first-pass metabolism. For example, if an oral dose has 60% bioavailability, only 60% of the dose reaches systemic circulation.