In first-order kinetics, the half-life depends on the percentage of drug remaining.

• A. Absolute amount in body
• B. Route of administration
• C. Molecular weight
• D. Percentage of drug remaining

Answer: (D)

First-order kinetics involve a constant fraction of drug eliminated per unit time, giving exponential decay. The half-life is the time required for the amount to fall to half of its initial value, meaning 50% remains. Because half-life is defined by reaching that specific percentage remaining, it is inherently tied to the percentage remaining. The actual value of the half-life comes from the rate constant (t1/2 = 0.693/k) and does not depend on the initial amount, though the concept of halving is itself a statement about a percentage change. Absolute amount in the body, route of administration, or molecular weight do not set the intrinsic half-life in simple first-order kinetics, though they can influence overall kinetics in more complex scenarios.