A drug undergoing zero-order (saturation) kinetics has a plasma concentration of 40 mg/L at time 0 and 30 mg/L at 2 hours. What will the plasma concentration be at 4 hours?

• A 22.5 mg/L
• B 20 mg/L ✓
• C 15 mg/L
• D 28 mg/L

Explanation

In zero-order kinetics, a constant amount (not fraction) of drug is eliminated per unit time. Between 0 and 2 hours, concentration fell from 40 to 30 mg/L — a drop of 10 mg/L per 2 hours (rate = 5 mg/L/hour). Applying this constant rate over the next 2 hours: 30 mg/L − 10 mg/L = 20 mg/L at 4 hours. Zero-order kinetics is seen with ethanol, phenytoin (at high doses), and aspirin (at toxic doses) when metabolic enzymes are saturated.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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