What is the area under the curve (AUC) and how is it used to compare formulations?

• A. AUC is the maximum concentration observed.
• B. AUC represents total systemic exposure; used to compare bioavailability and exposure across routes or brands.
• C. AUC is the time to reach Cmax.
• D. AUC is the half-life.

Answer: (B)

AUC represents the total amount of drug that reaches the systemic circulation over time, i.e., the overall exposure after a dose. It comes from integrating the concentration–time curve, giving a measure in units like ng·h/mL. This total exposure depends on how much drug is absorbed (bioavailability) and how quickly it is cleared from the body.

Because of that, AUC is used to compare formulations: when you give the same dose with different formulations or via different routes, the AUC tells you how much overall exposure each formulation produces. If two formulations yield similar AUCs, they deliver similar systemic exposure and thus similar bioavailability (assuming linear kinetics). If AUC differs, the absorbed fraction differs (or the clearance context differs). For reference, AUC is proportional to F × Dose / CL for extravascular dosing, and equals Dose / CL for intravenous dosing.

In contrast, other metrics like Cmax (maximum concentration), Tmax (time to reach Cmax), and half-life describe different aspects of the curve, not total exposure.