In first-order kinetics, which statement is true?

• A. The elimination rate is independent of concentration.
• B. The half-life is variable and depends on concentration.
• C. The elimination rate is proportional to drug concentration; half-life is constant.
• D. Clearance is fixed and does not influence half-life.

Answer: (C)

In first-order kinetics, the amount of drug eliminated per unit time is proportional to how much drug is present. This means if the concentration doubles, the elimination rate doubles, while the fraction of drug eliminated per unit time stays the same. Because the removal follows exponential decay, the half-life remains constant for a given patient when clearance and volume of distribution are constant. The relationship t1/2 = 0.693 × (Vd/CL) shows that half-life depends on clearance and volume of distribution, not on the current concentration. So saying the elimination rate is proportional to concentration and the half-life is constant accurately describes first-order kinetics. In contrast, zero-order kinetics involves a constant amount eliminated per unit time (not proportional to concentration), the half-life would vary with dose, and clearance directly influences half-life through the Vd/CL relationship.