Learning Objective

Understand the concept of first-order drug elimination, its kinetic characteristics, and how it differs from zero-order elimination.

First-Order Elimination Rate

First-order elimination occurs when a constant fraction of the drug is eliminated per unit time. Half-life (t₁/₂) is constant, regardless of plasma concentration. Graphically, plasma concentration decreases exponentially over time.

Example:

• Dose: 80 mg
• Elimination half-life: 4 h
• Interpretation: 50% of the drug is eliminated every 4 h (e.g., 80 → 40 → 20 → 10 mg)

Key Points

Notes:

• Most drugs follow first-order kinetics: the rate slows as plasma concentration falls.
• Zero-order occurs when elimination mechanisms are saturated, e.g., enzymes at Vmax.

Bridge to Renal Physiology

Activity