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Drug distribution is movement of a drug between blood and other body compartments. Perfusion, membrane permeability, tissue binding and plasma-protein binding jointly affect it. The apparent distribution volume iswhere $A$ is the amount in the body and $C_p$ the plasma concentration. If $A=100$ mg and $C_p=2$ mg/L, $V_d=50$ L. ^[ref]

The result is a proportionality quantity, not an anatomical cavity that must contain that many litres. Extensive tissue binding can lower $C_p$ and make $V_d$ large. A rapid fall in plasma concentration immediately after administration can therefore reflect distribution as well as drug elimination; distinguishing them requires a model and a suitable sampling schedule.

Drug elimination removes a parent drug from the body's measured parent-drug pool through drug metabolism or excretion. Redistribution from blood to a tissue changes plasma concentration but does not by itself eliminate the drug from the body. A mass balance separates input, output and transfers between compartments. ^[ref]

For a one-compartment model with no input and first-order elimination, $dA/dt=-kA$ gives $A(t)=A(0)e^{-kt}$. Equal fractions disappear in equal time intervals. By contrast, a saturated elimination route can approach removal of equal amounts per interval. The latter produces a different concentration-time curve and cannot be assumed to have a concentration-independent half-life.

Drug withdrawal is a physiological or behavioural response to reducing or stopping exposure after adaptation to a drug. It differs from simple elimination of the substance: its manifestations reflect the adapted system functioning under changed exposure. It also differs from recurrence of the original condition, although both can occur together. ^[ref]

For example, a system that has compensated for a persistent inhibitory influence may show excessive activity when that influence is removed. The timescale of adaptation can differ from the drug elimination timescale, so a response can continue after concentrations have fallen. Physical dependence, drug tolerance and compulsive use describe different phenomena; the presence of one alone does not establish all the others.