A drug has a volume of distribution (Vd) of 700 litres and a hepatic clearance of 35 L/hour. Calculate its elimination half-life.
- A 10 hours
- B 14 hours ✓
- C 20 hours
- D 28 hours
Correct answer: B. 14 hours
Explanation
Half-life (t½) = (0.693 × Vd) / CL = (0.693 × 700) / 35 = 485.1 / 35 = 13.86 ≈ 14 hours. A drug with a large Vd and moderate clearance will have a prolonged half-life because the drug is extensively distributed into tissues and must be 'retrieved' from those compartments for elimination. Drugs with very large Vd (hundreds of litres) are typically highly lipophilic and tissue-bound. Option A (10 h) would require CL ~48.5 L/h; option C (20 h) would require CL = 24.3 L/h; option D (28 h) would require CL = 17.3 L/h.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
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