What characterizes zero-order elimination?

• A. A constant amount of drug is cleared per unit time, independent of concentration.
• B. Clearance is proportional to concentration.
• C. Elimination rate slows down as concentration decreases.
• D. Elimination rate increases with time.

Answer: (A)

Zero-order elimination means the body clears a fixed amount of drug per unit time, regardless of how much drug is present. This happens when the elimination processes are saturated and operate at their capacity, so dA/dt is constant. As a result, the concentration declines linearly over time rather than exponentially. This is different from first-order elimination, where the rate of removal depends on how much drug is in the body and slows as concentration falls. The idea that the elimination rate increases with time isn’t characteristic of zero-order; that would imply a changing capacity or induction, not saturation. A classic example illustrating zero-order kinetics is alcohol metabolism at certain concentrations.