A drug is given intravenously at a loading dose of 100 mg. At steady state, the plasma concentration is measured as 2 mg/L. The volume of distribution (Vd) is:

• A 5 L
• B 200 L
• C 50 L ✓
• D 0.02 L

Explanation

Volume of distribution = Dose / Concentration = 100 mg ÷ 2 mg/L = 50 L. A Vd of 50 L suggests the drug distributes beyond plasma (5 L) and extracellular fluid (14 L) into tissues, indicating moderate tissue binding. A very high Vd (>500 L) would suggest extensive tissue sequestration (e.g., chloroquine); 5 L suggests confinement to plasma.

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

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