In pharmacokinetics, what term describes the fraction of a drug that reaches systemic circulation and is available to have an effect?

• A. The time it takes for a drug to start having an effect.
• B. The fraction of a drug that reaches systemic circulation and is available to have an effect.
• C. The maximum concentration of a drug in plasma after administration.
• D. The rate at which a drug is eliminated from the body.

Answer: (B)

Bioavailability is the fraction of a drug that reaches the systemic circulation and is available to have an effect. It captures how much of the administered dose becomes available to act after absorption and any first-pass metabolism. For an intravenous dose, bioavailability is effectively 100% because the drug goes directly into the bloodstream without barriers. With other routes, like oral, bioavailability is usually less than 1 due to incomplete absorption and metabolism before reaching the circulation. For example, if 100 mg is taken orally and only 50 mg enters the blood, the bioavailability is 0.5 (50%). The other concepts described in the choices relate to onset of action, peak concentration, and how fast the drug is eliminated, not to the fraction that actually reaches systemic circulation.