First-Order Elimination Kinetics is characterized by which statement?

First-Order Elimination Kinetics is characterized by which statement?

• A. The rate is proportional to the amount of drug in the body.
• B. The rate is constant regardless of concentration.
• C. The rate of elimination is proportional to the concentration of the drug.
• D. It does not apply to most drugs.

Answer: (C)

First-order elimination means the amount of drug the body removes per unit time changes with how much drug is present—the rate is a constant fraction of the current concentration. Mathematically, that’s dC/dt = -k C, so as concentration falls, the elimination rate falls too, producing a smooth, exponential decline and a constant half-life for a given clearance and volume of distribution. Because A (the amount) equals C times the volume of distribution, you can also say elimination is proportional to the amount remaining, but the standard way to express it is that the rate of elimination is proportional to the drug concentration. This is in contrast to zero-order kinetics, where the rate is constant regardless of concentration. And it’s false that first-order kinetics doesn’t apply to most drugs; at therapeutic levels, most drugs follow first-order behavior.

First-order elimination means the amount of drug the body removes per unit time changes with how much drug is present—the rate is a constant fraction of the current concentration. Mathematically, that’s dC/dt = -k C, so as concentration falls, the elimination rate falls too, producing a smooth, exponential decline and a constant half-life for a given clearance and volume of distribution. Because A (the amount) equals C times the volume of distribution, you can also say elimination is proportional to the amount remaining, but the standard way to express it is that the rate of elimination is proportional to the drug concentration. This is in contrast to zero-order kinetics, where the rate is constant regardless of concentration. And it’s false that first-order kinetics doesn’t apply to most drugs; at therapeutic levels, most drugs follow first-order behavior.