Which statement correctly describes how dose affects Cmax for first-order versus zero-order absorption?

• A. In zero-order absorption, Cmax increases with dose more than in first-order.
• B. In first-order absorption, Cmax increases with dose; in zero-order absorption, Cmax changes less with dose.
• C. In both, Cmax is unaffected by dose.
• D. In first-order absorption, Cmax is independent of dose.

Answer: (B)

Cmax reflects how fast and how much drug gets into the bloodstream relative to how quickly it’s cleared. With first-order absorption, the rate of entry is proportional to how much drug remains to be absorbed. So when you increase the dose, more drug is entering per unit time at all times, pushing the peak concentration higher. In contrast, zero-order absorption delivers drug at a constant rate regardless of how much is left to absorb. Increasing the total dose simply lengthens the period over which the drug is being absorbed, rather than increasing the instantaneous input rate. Since the body is eliminating during this extended absorption, the peak concentration doesn’t rise as much. So Cmax rises with dose in first-order absorption, but changes less with dose in zero-order absorption.