Elimination Kinetics: First-Order and Zero-Order

A drug that enters the body is eliminated in due course, primarily via the kidneys.

Clearance is a pharmacokinetic parameter, relating a drug's elimination rate to its plasma concentration. Typically, drug clearance follows first-order kinetics and depends on V_m, K_m, and drug concentration.

In first-order kinetics, a constant drug fraction is eliminated per unit time. The plot of drug concentration versus time is exponential, implying that a rise in the concentration increases its elimination rate.

However, the elimination of a few drugs, like phenytoin and aspirin, follows zero-order kinetics. Their high therapeutic doses saturate the enzyme's drug-metabolizing capacity, eliminating a fixed drug amount at a constant rate.

The resulting plot of drug concentration versus time becomes linear, and the elimination rate remains unaffected by the increasing drug concentration.