Which statement defines oral bioavailability F?

• A. The fraction of drug eliminated before reaching systemic circulation.
• B. The fraction of the administered dose that reaches systemic circulation.
• C. The fraction of dose excreted unchanged in urine.
• D. The fraction of drug absorbed but not reaching systemic circulation.

Answer: (B)

Oral bioavailability is the fraction of an orally administered dose that reaches systemic circulation in active form. It captures the combined effects of absorption from the GI tract and any loss due to first-pass metabolism in the gut wall and liver. By definition, this fraction is what would be available systemically, and it can be compared to an IV dose (for IV administration, bioavailability is essentially 1 because there is no absorption barrier). The other statements describe drug loss before reaching circulation (pre-systemic elimination), excretion in urine (renal clearance), or absorption that never reaches systemic circulation due to metabolism or incomplete absorption, none of which define the overall fraction that actually enters the bloodstream.