Half-life of distribution

Movement of drug from the plasma into the tissues as an equilibrium is established is a first-order process (in other words, movement is concentration-dependent and and is usually driven by a concentration gradient); the higher the concentration of drug in the plasma (or, more specifically, the greater the concentration gradient between the plasma and the tissues), the faster drug moves to the tissues. The mechanics of the process are similar to those which drive absorption, including the impact of drug lipid solubility.

Concentration-dependent distribution therefore has a first order rate constant for distribution (α) and, therefore, a half-life for distribution (0.693/α). The half-life for distribution is the time it takes for the drug concentration in the plasma to drop to a level midway between the initial concentration at time zero when absorption is complete (the A+B intercept on a two-compartment graph), and the plateau reached when distribution is complete but before any drug has been eliminated (the B-intercept on a two-compartment graph).