extent of distribution

If distribution describes the journey of a drug from blood into tissues, the extent of distribution describes how far that journey goes — whether the drug stays close to the blood or travels deep into the body's tissues. It is the breadth of a drug's reach beyond plasma.

Extent is shaped by lipophilicity (fat-soluble drugs penetrate further), molecular size and charge (large or highly charged drugs stay near the blood), plasma protein binding (which holds drug back in the blood) and tissue binding (which pulls it onward). The apparent volume of distribution is the usual numerical summary: a small value signals limited extent, a large value signals wide penetration into tissues.

Extent of distribution has real clinical weight. A drug that distributes poorly may fail to reach an infection deep in tissue, bone or the central nervous system even when blood levels look adequate; a drug that distributes very widely may need a larger loading dose and resist removal by dialysis. Knowing where a drug does and does not go helps predict both efficacy and safety.