What term describes the amount of drug that actually reaches systemic circulation?

The term that describes the amount of drug that actually reaches systemic circulation is bioavailability. Bioavailability is a crucial pharmacokinetic property that reflects the extent and rate at which the active ingredient or active moiety is absorbed and becomes available at the site of action.

Understanding bioavailability helps in assessing how effectively a drug can exert its intended effects once administered. For example, when a medication is taken orally, various factors such as the form of the drug, the presence of food in the stomach, and the metabolism of the drug can all influence how much of the drug actually enters systemic circulation.

In contrast, potency refers to the amount of drug needed to produce a given effect, while efficacy is related to the maximum effect that a drug can produce. Absorption specifically refers to the process of the drug moving from the site of administration into the bloodstream, but it does not account for how much of that drug is ultimately available for therapeutic effect. Thus, bioavailability uniquely encapsulates the complete picture of a drug's availability in systemic circulation, making it the correct term in this context.