Which term describes how well a drug is absorbed and becomes available at the site of action?

• A. Bioavailability
• B. Potency
• C. Therapeutic Level
• D. Toxicity

Answer: (A)

The main concept tested is how much of a drug actually makes it into the bloodstream and becomes available to exert its effect. This is bioavailability. It describes the fraction of the administered dose that reaches systemic circulation intact, and thus the amount that can reach the site of action. Route of administration matters: intravenous administration delivers the full dose directly into circulation, giving essentially 100% bioavailability, while oral administration often yields less because of incomplete absorption and first-pass metabolism in the liver.

Understanding this helps distinguish other terms: potency is about the amount of drug needed to produce a given effect (how strong the drug is), not how much of it reaches the bloodstream. Therapeutic level refers to the concentration range in the blood that provides efficacy, not the absorption process itself. Toxicity concerns harmful effects and is related to concentration, not the fraction absorbed.