A pharmacokinetic parameter (abbreviated Vd) that describes how extensively a drug distributes from the bloodstream into body tissues. It is not a literal physical volume—it is a mathematical ratio of the total amount of drug in the body to its blood concentration.

A small Vd (close to blood volume, ~5 L) means the drug stays mainly in the bloodstream. A large Vd (hundreds of liters) means the drug concentrates heavily in tissues. Peptides generally have relatively small volumes of distribution because their size limits tissue penetration, but modifications like PEGylation or lipidation can alter distribution significantly. Vd helps predict how long a peptide will remain active and whether it will reach target tissues.