Bioavailability is defined as the ratio of drug absorbed from the administered dose.

• A. The portion of the dose that reaches systemic circulation
• B. The rate of drug metabolism in the liver
• C. The time to reach peak concentration
• D. The volume of distribution

Answer: (A)

Bioavailability tells you how much of the administered dose actually becomes available in the systemic circulation to exert an effect. It is the fraction of the dose that reaches the bloodstream in active form, accounting for absorption and any first-pass metabolism that reduces the amount before it gets there. That’s why the best description is the portion of the dose that reaches systemic circulation. The rate of metabolism in the liver describes how quickly the drug is cleared, not how much enters the blood. The time to reach peak concentration relates to how fast the drug is absorbed and distributed, not the total amount available. The volume of distribution is a theoretical space that links amount in the body to blood concentration and does not measure the fraction that reaches circulation.