A drug has a volume of distribution (Vd) of 700 L in a 70 kg adult. This large Vd implies that:

• A The drug is highly protein bound in plasma, restricting it to the vascular compartment
• B The drug has a high hepatic extraction ratio, ensuring rapid first-pass metabolism before systemic distribution
• C The drug extensively partitions into deep tissue compartments (fat, muscle) or is bound to tissue proteins, leaving very little in the plasma compartment relative to total body drug ✓
• D The drug equilibrates only with total body water (42 L) plus adipose tissue water

Explanation

Volume of distribution (Vd = total drug in body / plasma drug concentration) is a conceptual volume representing how extensively a drug distributes outside the plasma. Normal Vd values: ~5 L (plasma only, highly protein-bound drugs like heparin), ~15 L (ECF only), ~42 L (total body water), >100 L (extensive tissue binding or lipophilic — e.g., amiodarone ~5000 L, chloroquine ~200–500 L). A Vd of 700 L means that 700 L of plasma would be needed to account for all drug at the measured plasma concentration — indicating the vast majority is in tissues. This has clinical implications: dialysis is ineffective for overdose (drug is not in plasma), and loading doses must account for the large Vd.

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

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Written and medically reviewed by the StethoPrep medical team.