What defines 'first-order kinetics' in drug elimination?

First-order kinetics in drug elimination is characterized by the principle that the rate of drug elimination is proportional to the plasma concentration of the drug. This means that when the concentration of the drug is higher, the rate at which it is eliminated from the body also increases. This relationship typically manifests in a predictable, exponential decay of the drug concentration over time, allowing for a straightforward mathematical modeling of drug clearance and half-life.

In contrast, a constant rate of elimination, regardless of concentration, describes zero-order kinetics, where the drug is eliminated at a fixed rate. Variability with different dosage forms refers to how the formulation of the drug might affect the absorption or release of the drug rather than its elimination kinetics. Lastly, while enzymatic activity can influence drug metabolism and elimination, it does not define the specific nature of first-order kinetics but rather can affect how quickly a drug is processed in both first-order and zero-order kinetics scenarios. Overall, the defining aspect of first-order kinetics is the proportionality of elimination rate to the drug concentration in the plasma.