Glossary and Definitions Drug distribution is the process by which a drug reversibly leaves the blood stream and enters the interstitium (extracellular fluid) and/or the cells of the tissues. The delivery of a drug from the plasma to the interstitium primarily depends on blood flow, capillary permeability, the degree of binding of the drug to plasma and tissue proteins, and the relative hydrophobicity of the drug. Excipient: Vehicle. A more or less inert substance added in a prescription as a diluent or vehicle or to give form or consistency when the remedy is given in pill form; simple syrup, aromatic powder, honey, and various elixirs are examples. Gel: A colloidal st