A pharmacokinetic parameter that relates the total amount of drug in the body to the concentration in the bloodstream.

Vd is defined as the total amount of drug in the body divided by its concentration in plasma. Thus, Vd reflects the degree to which the drug is present in extravascular tissues rather than in the plasma. A drug with a high Vd tends to leave the plasma and enter other compartments in the body, leading to low plasma concentrations. A drug with a low Vd tends to remain in the plasma, meaning a lower dose of a drug is required to achieve a given plasma concentration.

Vd is dependent on both the chemical properties of a drug (e.g. highly lipid-soluble drugs have good cell penetration, resulting in high Vd, while drugs which bind to plasma proteins such as albumin have a reduced Vd) and patient physiology. Clinically, Vd is of most significance for determining an initial loading dose of an antibiotic, assuming that successful therapy is directly linked to its plasma concentration.