Part of the pharmacokinetics.

Theoretical volume occupied by the total amount of drug in the body relative to its plasma concentration. Apparent V_d of plasma protein-bound drugs can be altered by liver and kidney disease (↓ protein binding, ↑ V_d). Drugs may distribute in more than one compartment.

V_d = amount of drug in the body/plasma drug concentration

Low V_d: Intravascular- Large/charged molecules- Plasma protein boundMedium V_d: Extracellular fluid- Small hydrophilic moleculesHigh V_d: All tissues including fat- Small lipophilic molecules, especially if bound to tissue protein