Which statement correctly describes a drug that follows first-order elimination kinetics?

• A A constant amount of drug is eliminated per unit time regardless of plasma concentration
• B The elimination half-life remains constant irrespective of the plasma concentration ✓
• C The plasma concentration declines linearly with time on a linear plot
• D Elimination rate increases disproportionately as plasma concentration rises

Explanation

In first-order kinetics, a constant fraction of drug is eliminated per unit time, so the half-life is constant and independent of concentration. Option A describes zero-order kinetics, where a constant amount is removed per unit time. Option C is wrong because first-order decline is exponential, appearing linear only on a semilogarithmic plot. Option D describes saturable, capacity-limited behaviour seen when elimination mechanisms approach saturation.

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

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