Extravascular administration, such as oral or intramuscular routes, is a non-invasive drug delivery method, often preferred for ease and patient compliance. A key factor here is absorption, which dictates how quickly and effectively the drug enters the bloodstream from the administration site. Absorption follows either zero-order or first-order kinetics.

Zero-order absorption maintains a steady rate irrespective of the amount of drug left to be absorbed, making it a constant process. In the zero-order absorption model, the rate of drug absorption remains constant over time, akin to the constant-rate infusion model. This type of kinetics often applies to drugs delivered via controlled-release systems, such as sustained-release tablets or transdermal patches. Here, absorption continues steadily until the drug at the absorption site (e.g., gastrointestinal tract) is depleted. The mathematical equations and principles governing plasma concentration-time profiles in constant-rate intravenous infusion also apply to zero-order absorption.

On the contrary, first-order absorption sees a declining rate as the amount to be absorbed decreases. In the context of one-compartment open models for extravascular administration, the change in drug amount in the body is determined by the balance between absorption and elimination rates. Pharmacokinetic parameters like elimination rate constant (k), absorption rate constant (k_a), maximum plasma concentration (C_max), and time to reach maximum concentration (t_max) can be estimated using suitable equations, providing critical insights into drug behavior and effectiveness.