Which statement best characterises first-order elimination kinetics?

• A A constant amount of drug is eliminated per unit time
• B A constant fraction of the drug present is eliminated per unit time ✓
• C The plasma half-life shortens as concentration falls
• D Plasma concentration declines linearly with time on arithmetic plot

Explanation

First-order elimination means elimination rate is proportional to concentration, so a fixed fraction (not amount) of drug is removed per unit time and the half-life remains constant regardless of dose. A constant amount per unit time defines zero-order kinetics. On an arithmetic plot, first-order decline is exponential and curved; it becomes linear only on a semilogarithmic plot. Most drugs follow first-order kinetics at therapeutic concentrations.

Reference: Katzung's Basic and Clinical Pharmacology, 15th ed.

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