Which term refers to the percentage of a drug that enters the systemic circulation?

The term that refers to the percentage of a drug that enters the systemic circulation is bioavailability. Bioavailability is a pharmacokinetic parameter that indicates the extent and rate at which the active ingredient or active moiety is absorbed and becomes available at the site of action. This concept is crucial for understanding how much of a drug is effectively utilized by the body after administration, which can vary significantly depending on the route of administration (oral, intravenous, etc.).

For instance, when a drug is administered intravenously, its bioavailability is 100% because it is directly introduced into the bloodstream. In contrast, medications taken orally may have lower bioavailability due to factors such as first-pass metabolism in the liver or incomplete absorption from the gastrointestinal tract. Understanding bioavailability helps healthcare practitioners determine the appropriate dosing and expected therapeutic effects of a medication.

Absorption is related but specifically refers to the process by which a drug enters the bloodstream. Distribution covers how the drug disperses throughout the body once in circulation, and metabolism pertains to the biochemical alterations a drug undergoes within the body, ultimately affecting its efficacy and elimination. Thus, while these terms are interconnected in the pharmacokinetic processes, bioavailability specifically quantifies the fraction of the drug that successfully reaches systemic circulation.