Which statement best describes zero-order elimination?

Which statement best describes zero-order elimination?

• A. A fixed amount is eliminated per unit time.
• B. A fixed fraction is eliminated per unit time.
• C. Elimination rate increases as concentration increases.
• D. Elimination rate decreases as concentration increases.

Answer: (A)

Zero-order elimination means the body clears a constant amount of drug per unit time because the elimination pathways are saturated and operate at capacity. In this regime, the rate is independent of how much drug is present, so doubling the concentration doesn’t speed up clearance until you drop below the saturation point. That’s why the best description is a fixed amount eliminated per unit time. When the drug concentration is high enough to saturate the enzymes, elimination proceeds at Vmax and is not tied to concentration, which contrasts with first-order elimination where a constant fraction is removed per unit time and the rate increases with concentration. Clinically, zero-order kinetics can lead to disproportionate increases in plasma levels with small dose increases and yields a changing half-life as the concentration changes within the saturable range.

Zero-order elimination means the body clears a constant amount of drug per unit time because the elimination pathways are saturated and operate at capacity. In this regime, the rate is independent of how much drug is present, so doubling the concentration doesn’t speed up clearance until you drop below the saturation point. That’s why the best description is a fixed amount eliminated per unit time. When the drug concentration is high enough to saturate the enzymes, elimination proceeds at Vmax and is not tied to concentration, which contrasts with first-order elimination where a constant fraction is removed per unit time and the rate increases with concentration. Clinically, zero-order kinetics can lead to disproportionate increases in plasma levels with small dose increases and yields a changing half-life as the concentration changes within the saturable range.