AUC

Plot a drug's blood concentration against time and you get a hump that rises, peaks, and tails off. AUC is simply the area enclosed under that hump. Where Cmax captures only the single highest point, AUC sums up the whole exposure — how much drug, for how long — into one number that represents the body's total dose received over time.

Quantitatively, AUC is the integral of plasma concentration with respect to time, usually computed from sampled data by the trapezoidal rule, with units of concentration × time (for example ng·h/mL). AUC from time zero to the last sample (AUC_0–t) is often extrapolated to infinity (AUC_0–∞). It is the principal measure of the extent of absorption and of overall systemic exposure, and it scales with dose for drugs that follow linear kinetics.

AUC is the workhorse of bioequivalence and bioavailability: the ratio of AUCs between a test and reference product (dose-corrected) is the very definition of relative bioavailability, and a test-versus-IV AUC ratio gives absolute bioavailability. Because it reflects total exposure rather than peak intensity, AUC ties closely to the overall efficacy and cumulative effect of a drug, complementing the rate information carried by Cmax and Tmax.