What does the concentration-time graph look like for first-order versus zero-order elimination?

• A. First-order: straight-line; Zero-order: exponential decline
• B. First-order: exponential decline; Zero-order: straight-line decline
• C. First-order: constant; Zero-order: decreasing
• D. First-order: increasing; Zero-order: constant

Answer: (B)

How the concentration of a drug falls over time depends on the order of elimination. In first-order elimination, the removal rate is proportional to how much drug is present, so the concentration-time profile follows an exponential decay: C = C0 e^{-kt}. This means the curve is curved—steepest at the start and then it gradually levels off as time goes on.

In zero-order elimination, the drug is removed at a constant amount per unit time, so the concentration declines linearly: C = C0 − k0 t. On a concentration-versus-time graph, this is a straight line with a constant negative slope, at least until the drug concentration hits zero or the elimination mechanism changes.

Therefore, the correct description is that first-order elimination shows an exponential decline, while zero-order elimination shows a straight-line decline.