What term describes how much of a drug is available to produce a biologic response?

• A. Potency
• B. Efficacy
• C. Bioavailability
• D. Tolerance

Answer: (C)

Biology's concept here is bioavailability—the fraction of a drug that reaches systemic circulation in active form after administration. This determines how much of the dose can actually produce a biological response. For instance, when a drug is given intravenously, nearly all of it enters circulation, so bioavailability is essentially 100%. Other routes, like oral, often have lower bioavailability due to incomplete absorption and first-pass metabolism, which reduces the amount that can act at target tissues.

In contrast, potency refers to the amount of drug needed to produce a given effect, not how much gets into circulation. Efficacy is about the maximal effect a drug can produce, regardless of dose. Tolerance involves a decreased response over time with repeated use. So the term describing how much of a drug is available to produce a biologic response is bioavailability.