Volume of Distribution

Volume of distribution is defined by division: the amount of drug in the body divided by the concentration measured in plasma. It is apparent rather than anatomical, and it answers one question, which is how much of the drug is somewhere other than plasma. A drug confined to the circulation shows a volume near plasma volume; one that concentrates in fat or binds intracellular proteins can show hundreds of litres, far more than the person containing it.

For reference, an adult of about 70 kilograms holds roughly 3 litres of plasma, 15 litres of extracellular fluid and 42 litres of total body water. Most therapeutic peptides are hydrophilic and too large to cross membranes readily, so they stay close to the extracellular space, with reported values in the region of 10 to 15 litres for the acylated GLP-1 analogues. Amiodarone and hydroxychloroquine sit at the other extreme, in the thousands of litres.

The parameter earns its place in two calculations. With clearance it produces half-life, since half-life is ln2 multiplied by volume and divided by clearance, which is why a drug can be long-lived through wide distribution rather than slow elimination. On its own it sizes a loading dose. A small volume also warns that tissue penetration is limited, which bears directly on claims of central nervous system activity.

Reading it as anatomy is the standard mistake; the number is a ratio, and 500 litres does not locate the drug anywhere. The second trap is comparing volumes across routes. After subcutaneous or oral dosing what is reported is volume divided by bioavailability, so a product with poor absorption shows an inflated apparent volume that reflects the fraction absorbed rather than any distribution property.