The elimination rate constant K or K_e is a value used in pharmacokinetics to describe the rate at which a drug is removed from the human system.

It is often abbreviated K or K_e. It is equivalent to the fraction of a substance that is removed per unit time measured at any particular instant and has units of T⁻¹. This can be expressed mathematically with the differential equation

where ${\displaystyle C_{t}}$ is the blood plasma concentration of drug in the system at a given point in time ${\displaystyle t}$, ${\displaystyle dt}$ is an infinitely small change in time, and ${\displaystyle C_{t+dt}}$ is the concentration of drug in the system after the infinitely small change in time.

The solution of this differential equation is useful in calculating the concentration after the administration of a single dose of drug via IV bolus injection:

In first-order (linear) kinetics, the plasma concentration ${\displaystyle C_{t}}$ of a drug at a given time t after single dose administration via IV bolus injection is given by;

${\displaystyle C_{t}={\frac {C_{0}}{2^{\frac {t}{t_{1/2}}}}}\,}$

where:

Therefore, the amount of drug present in the body at time t ${\displaystyle A_{t}}$ is;