For a drug following first-order elimination, the shape of the concentration-time curve remains identical when dose is scaled by 1x, 10x, or 100x for the same route of administration.

• A. Only at low doses
• B. Only if clearance changes.
• C. True
• D. False

Answer: (C)

In linear pharmacokinetics with first-order elimination, the rate of drug removal is proportional to how much is present, giving an exponential decline with a constant half-life. The concentration-time profile can be written as C(t) = (Dose/Vd) · e^(-k t) for a given route and clearance. If you scale the dose by any factor (1x, 10x, 100x), you simply scale the initial concentration but the exponential decay over time remains governed by the same k. So the shape of the curve is unchanged; only the height changes. This dose-proportional behavior also means AUC scales with dose, and the time course preserves its form as long as kinetics stay linear and the route remains the same. If kinetics were not linear (saturation) or absorption/ disposition changed with dose, the shape could differ.