In first-order elimination kinetics, which statement best describes the process?

In first-order elimination kinetics, which statement best describes the process?

• A. The amount of drug eliminated per unit time is constant.
• B. A constant fraction of drug is eliminated per unit time.
• C. The elimination rate decreases as dose increases.
• D. The half-life is directly dependent on the dose.

Answer: (B)

In first-order elimination, the amount of drug removed is proportional to how much is present in the body. This means a constant fraction of the drug is eliminated per unit of time. If you start with more drug, more is eliminated in the same time interval, but the fraction removed stays the same. Mathematically, this is described by dA/dt = -kA, leading to exponential decay and a half-life that does not depend on the dose (t1/2 = 0.693/k). That constant fraction behavior is the hallmark of first-order kinetics, which is why it’s the best description. The other statements don’t fit: a constant amount removed per unit time describes zero-order kinetics; the elimination rate does not decrease simply because the dose is higher (the rate scales with the amount present), and in first-order kinetics the half-life is not dose-dependent.

In first-order elimination, the amount of drug removed is proportional to how much is present in the body. This means a constant fraction of the drug is eliminated per unit of time. If you start with more drug, more is eliminated in the same time interval, but the fraction removed stays the same. Mathematically, this is described by dA/dt = -kA, leading to exponential decay and a half-life that does not depend on the dose (t1/2 = 0.693/k). That constant fraction behavior is the hallmark of first-order kinetics, which is why it’s the best description.

The other statements don’t fit: a constant amount removed per unit time describes zero-order kinetics; the elimination rate does not decrease simply because the dose is higher (the rate scales with the amount present), and in first-order kinetics the half-life is not dose-dependent.