Drug elimination

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.