elimination half-life

The elimination half-life answers a simple question with a memorable number: how long does it take for the drug concentration in the blood to drop to half of what it was? If a level halves in four hours, then four hours later it has halved again, and so on, stepping down in steady halvings.

This neat halving behaviour appears because most drugs are eliminated by first-order kinetics, where a constant fraction is removed per unit time rather than a constant amount. The half-life is determined jointly by clearance and the volume of distribution: it lengthens if the drug spreads widely into tissues, and shortens if the body clears it faster.

Half-life governs two practical things. It tells you how long elimination takes after stopping the drug, since after about four to five half-lives roughly 94 to 97 percent has gone. And it guides the dosing interval, because choosing an interval close to the half-life keeps the swing between peak and trough concentrations reasonable. The neat rule fails for drugs cleared by saturable, zero-order kinetics, whose half-life is not constant.