A drug with zero-order kinetics and a rate of elimination of 50 mg/hour is given at 600 mg IV bolus. How many hours will it take to reach zero plasma concentration?
- A 8 hours
- B 18 hours
- C 12 hours ✓
- D 6 hours
Correct answer: C. 12 hours
Explanation
In zero-order (saturation) kinetics, a constant amount of drug is eliminated per unit time, independent of plasma concentration. Time to elimination = Total dose / Rate of elimination = 600 mg / 50 mg/hr = 12 hours. Examples of drugs following zero-order kinetics at clinical doses include phenytoin (at high concentrations), ethanol, and aspirin (in overdose). Unlike first-order kinetics where half-life is constant, zero-order elimination means that higher doses take proportionally longer to clear, increasing toxicity risk with small dose increments.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
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