Drug absorption via extravascular administration
If a drug is given by any other route than directly into the plasma, it is considered extravascular and must therefore undergo absorption. Absorption is a process that usually depends on the concentration: When higher drug concentrations reach the absorption site, more drug is absorbed. In fact, a constant fraction (percentage) is absorbed. This concentration-dependent (absorption) rate is called first-order absorption.
The principles and practical consequences of first-order kinetics is more in depth explained in the topic elimination, as elimination is also (often) a first-order process.
After as drug enters the circulation, it is also eliminated. At some point, the absolute amount of drug offered to the systemic circulation is diminishing (because most of it is now absorbed). This causes a flattening of the concentration time curve and it will reach maximum concentration called Cmax at time tmax. At this point the rate at which the drug enters the circulation is exactly the same as the rate at which it leaves the circulation through clearance and this is analogous to an intravenous infusion. It is also possible that not all of the drug reaches the systemic circulation (see bioavailability).