The first-order absorption model for extravascular administration describes the rate at which a drug is absorbed and eliminated, following the principles of first-order kinetics. This model is vital as it provides a mathematical representation of drug behavior within the body. It also allows for the prediction and interpretation of drug absorption and elimination based on the rate of change in drug concentration over time. This model can be visualized as a plasma concentration-time profile comprising absorption, post-absorption, and elimination phases. Crucial pharmacokinetic parameters such as peak plasma concentration (C_max) and the duration required to reach it (t_max) can be calculated using this model.

Methods like residuals or the Wagner-Nelson method can be employed to determine the absorption rate constant. If the extrapolated and residual lines intersect at the y-axis at time t = 0, then there is no lag in absorption. However, if the intersection occurs at a time greater than zero, this indicates a time lag. The time lag is defined as the time difference between drug administration and the beginning of the absorption process. It is denoted by the symbol t₀ and represents the start of absorption.

An interesting phenomenon that may occur in the plasma concentration-time curve is the flip-flop phenomenon. This refers to an interchange in the meanings of the slopes representing absorption and elimination. During the flip-flop phenomenon, a longer duration of drug sampling might be necessary to avoid overestimating the fraction of drug absorbed.