Bioavailability refers to which of the following?

Bioavailability refers to which of the following?

• A. The fraction of an administered dose that reaches systemic circulation
• B. The time it takes to feel an effect
• C. The speed of excretion
• D. The degree of protein binding

Answer: (B)

Bioavailability is the portion of the administered dose that actually reaches the bloodstream and is available to have an effect. This is why it’s a measure of how much of the drug is absorbed and escapes any first-pass metabolism before reaching systemic circulation. It explains why different routes of administration yield different amounts of drug in the blood—for example, an intravenous dose reaches the bloodstream almost entirely, while an oral dose often results in less drug entering the circulation due to absorption barriers and liver metabolism. The time to feel an effect describes onset of action, which is about speed rather than how much drug ultimately reaches circulation. The speed of excretion refers to clearance, and the degree of protein binding affects distribution and free drug available at the site of action, not the fraction that reaches systemic circulation.

Bioavailability is the portion of the administered dose that actually reaches the bloodstream and is available to have an effect. This is why it’s a measure of how much of the drug is absorbed and escapes any first-pass metabolism before reaching systemic circulation. It explains why different routes of administration yield different amounts of drug in the blood—for example, an intravenous dose reaches the bloodstream almost entirely, while an oral dose often results in less drug entering the circulation due to absorption barriers and liver metabolism. The time to feel an effect describes onset of action, which is about speed rather than how much drug ultimately reaches circulation. The speed of excretion refers to clearance, and the degree of protein binding affects distribution and free drug available at the site of action, not the fraction that reaches systemic circulation.