How does the half-life change in first-order versus zero-order kinetics?

• A. First-order: half-life constant; Zero-order: not constant and increases with concentration.
• B. First-order: half-life increases; Zero-order: constant
• C. First-order: half-life constant; Zero-order: not constant and increases with concentration.
• D. First-order: half-life decreases; Zero-order: decreases

Answer: (A)

Half-life behavior depends on how the elimination rate relates to concentration. In first-order kinetics, the rate is proportional to the concentration, leading to exponential decay. The time it takes to cut the concentration in half is constant and given by t1/2 = 0.693/k, independent of how much drug you started with. In zero-order kinetics, the rate is constant and does not depend on concentration, so the concentration falls linearly over time. To reduce from an initial concentration C0 to C0/2, you must remove C0/2 at a constant rate, which takes t1/2 = (C0/2)/k0 time. Thus, the half-life is not constant and increases with higher initial concentrations. Therefore, the statement that first-order half-life is constant and zero-order half-life is not constant and increases with concentration is correct.