What happens to the elimination rate in first-order kinetics as drug concentration falls?

What happens to the elimination rate in first-order kinetics as drug concentration falls?

• A. It becomes constant
• B. It decreases proportionally with concentration
• C. It increases
• D. It becomes zero

Answer: (B)

In first-order kinetics, the rate of elimination is proportional to how much drug is present. This is described by dA/dt = -k·A (or dC/dt = -k·C). Because the amount in the body drives the rate, as concentration falls, the elimination rate falls in the same proportion. If the concentration halves, the amount eliminated per unit time also halves. The fraction eliminated per unit time remains constant (a constant k), but the actual amount removed per hour decreases as concentration declines. This is different from zero-order kinetics, where the elimination rate stays the same regardless of concentration.

In first-order kinetics, the rate of elimination is proportional to how much drug is present. This is described by dA/dt = -k·A (or dC/dt = -k·C). Because the amount in the body drives the rate, as concentration falls, the elimination rate falls in the same proportion. If the concentration halves, the amount eliminated per unit time also halves. The fraction eliminated per unit time remains constant (a constant k), but the actual amount removed per hour decreases as concentration declines. This is different from zero-order kinetics, where the elimination rate stays the same regardless of concentration.